There is no FTP access to free hosted WordPress.com blogs and we bloggers cannot install any plugins. They are only for WordPress.org installs. There is no upgrade you can purchase that changes that reality.
http://support.wordpress.com/com-vs-org/

You can use any widget that has pure HTML but you cannot use any widgets that contain restricted codes like iframes or JavaScript widgets. In the case of Pinterest, you cannot use the Pin-it Javascript code on a WordPress.com blog. http://en.support.wordpress.com/code/

But you can put the code for the Pinterest follow button in a text widget.
1. Go to this site, choose which one of the red Follow Me images you want, and copy the code for it. http://pinterest.com/about/goodies/

2. Then, go to your widgets > Appearance > Widgets and select a text widget.http://en.support.wordpress.com/widgets/text-widget/

3. Paste the Pinterest follow button code into the text widget, "save" the widget, "close" the widget and place it where you want it to display in your sidebar.
http://en.support.wordpress.com/widgets/text-widget/
http://en.support.wordpress.com/links/text-widget-links/
http://en.support.wordpress.com/widgets/text-widget/#adding-an-image

4. Please see here for enabling sharing buttons including Pinterest on your blog. http://en.support.wordpress.com/sharing/

/* Copyright 2012 Christopher Ross ( email : [email redacted] )

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
( at your option ) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*/

class thisismyurl_easy_pinterest_widget extends WP_Widget
{

function thisismyurl_easy_pinterest_widget() {
$widget_ops = array( 'classname' => 'widget_thisismyurl_easy_pinterest',
'description' => __( "A WordPress widget to add your recent Pinterest posts to your WordPress website. Learn more at http://thisismyurl.com" ) );
$control_ops = array( 'width' => 300, 'height' => 300 );
$this->WP_Widget( 'thisismyurl_easy_pinterest_widget', __( 'Easy Pinterest' ), $widget_ops, $control_ops );
}

function update( $new_instance, $old_instance ) {
$instance = $old_instance;
$instance['title'] = strip_tags( stripslashes( $new_instance['title'] ) );
$instance['pinterest_username'] = strip_tags( stripslashes( $new_instance['pinterest_username'] ) );
$instance['pinterest_quantity'] = strip_tags( stripslashes( $new_instance['pinterest_quantity'] ) );

return $instance;
}

function form( $instance ) {
$instance = wp_parse_args( ( array ) $instance, array( 'title'=>'Pinterest Activity', 'pinterest_username'=>'thisismyurl', 'pinterest_quantity'=>'12' ) );

$title = htmlspecialchars( $instance['title'] );
$pinterest_username = ( $instance['pinterest_username'] );
$pinterest_quantity = ( $instance['pinterest_quantity'] );

echo '<p style="text-align:left;"><label for="' . $this->get_field_name( 'title' ) . '">' . __( 'Title:' ) . '</label>
<input style="width: 300px;" id="' . $this->get_field_id( 'title' ) . '" name="' . $this->get_field_name( 'title' ) . '" type="text" value="' . $title . '" /></p>';

echo '<p style="text-align:left;"><label for="' . $this->get_field_name( 'pinterest_username' ) . '">' . __( 'Pinterest Username:' ) . '</label>
<input style="width: 300px;" id="' . $this->get_field_id( 'pinterest_username' ) . '" name="' . $this->get_field_name( 'pinterest_username' ) . '" type="text" value="' . $pinterest_username . '" /></p>';

echo '<p style="text-align:left;"><label for="' . $this->get_field_name( 'pinterest_quantity' ) . '">' . __( '# to Show:' ) . '</label>
<input style="width: 300px;" id="' . $this->get_field_id( 'pinterest_quantity' ) . '" name="' . $this->get_field_name( 'pinterest_quantity' ) . '" type="text" value="' . $pinterest_quantity . '" /></p>';

}

function widget( $args, $instance ) {

extract( $args );
$instance = wp_parse_args( ( array ) $instance, array( 'title'=>'Pinterest Activity', 'pinterest_username'=>'thisismyurl', 'pinterest_quantity'=>'12') );

$pinterest_feed = fetch_feed( "http://pinterest.com/" . $instance['pinterest_username'] . "/feed.rss" );

if (!is_wp_error( $pinterest_feed ) ) :
$maxitems = $pinterest_feed->get_item_quantity( $instance['pinterest_quantity'] );
$pinterest_feed = $pinterest_feed->get_items(0, $maxitems);
endif;

if ( !empty( $pinterest_feed ) ) {

echo $before_widget;

echo '<h4 class="widgettitle pinterest-title"><a href="http://pinterest.com/' . $instance['pinterest_username'] . '"
target="_blank">' . $instance['title'].'</h4>';

echo '<div class="content"><div class="textwidget">';
echo '<ul class="easy-pinterest">';
foreach ( $pinterest_feed as $item ) {
$pinterest_content = $item->get_content();
$pinterest_content = str_replace( '>','>',$pinterest_content );
$pinterest_content = str_replace( '<','<',$pinterest_content );
$pinterest_content = str_replace( '<a','<a target="_blank"',$pinterest_content );

$pinterest_content = strip_tags($pinterest_content, ",<img>");
$pinterest_content_array = explode("", $pinterest_content);
$pinterest_content = $pinterest_content_array[0];

?>

get_permalink() ); ?>'
title='<?php echo 'Posted '.$item->get_date('j F Y | g:i a'); ?>'><?php echo $pinterest_content; ?>

echo "";

$options = get_option( 'thisismyurl_easy_pinterest' );
$options_thankyou = $options->easy_pinterest_thankyou;

if ( $options_thankyou == "1")
echo '<p class="easy-pinterest-credit"><a href="http://thisismyurl.com/downloads/wordpress-plugins/"
title="Easy Pinterest Plugins for WordPress"
target="_blank">Easy Pinterest by thisismyurl.com</p>';

echo $after_widget;
}
}

}

function thisismyurl_easy_pinterest_css() {
echo "<!-- Easy Pinterest by Christopher Ross (http://thisismyurl.com) --><style>

h4.pinterest-title {background: url(" . plugins_url( 'images/pinterest-red.png' , __FILE__ ) . ") no-repeat right; padding-top:20px;}
ul.easy-pinterest ol {float:left; width: 90px; height: 90px; overflow: hidden; margin-right: 10px; margin: 0px 10px 10px 0px; background: #efefef;display:table-cell; vertical-align:middle;}
ul.easy-pinterest ol img {max-width: 100%; height: auto;}
ul.easy-pinterest ol p {display: none;}

p.easy-pinterest-credit {font-size: .5em; opacity: .5;}

</style>";
}
add_action( 'wp_head', 'thisismyurl_easy_pinterest_css' );

function thisismyurl_easy_pinterest_widget_Init() {
register_widget( 'thisismyurl_easy_pinterest_widget' );
}
add_action( 'widgets_init', 'thisismyurl_easy_pinterest_widget_Init' );

// add menu to WP admin

function thisismyurl_easy_pinterest_menu() {add_options_page( __( 'Easy Pinterest' ), __( 'Easy Pinterest' ), 10,'thisismyurl_easy_pinterest.php', 'thisismyurl_easy_pinterest_options' );}
add_action( 'admin_menu', 'thisismyurl_easy_pinterest_menu' );

function thisismyurl_easy_pinterest_options( $options='' ) {

$options = get_option( 'thisismyurl_easy_pinterest' );

// save page options
if ( $_GET['page'] == 'thisismyurl_easy_pinterest.php' && !empty( $_POST['easy_pinterest_update'] ) ) {

$options->easy_pinterest_block = $_POST['easy_pinterest_block'];
$options->easy_pinterest_thankyou = $_POST['easy_pinterest_thankyou'];

update_option( 'thisismyurl_easy_pinterest',$options );
$options = get_option( 'thisismyurl_easy_pinterest' );
}

?>

<div class="wrap">

<div class='plugin_header' style='background:#EEF0F7; margin-top: 20px; padding: 20px; -moz-border-radius: 5px; border-radius: 5px;'>
<div style='float:left; width: 55%;'>
<h2><?php _e( 'Easy Pinterest by Christopher Ross','easy_pinterest' ) ?></h2>
<p>An easy way to add recent Pinterest posts to your WordPress website as a widget.</p>
<form name="_xclick" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="[email redacted]">
<input type="hidden" name="item_name" value="Donation for <?php echo $cr_wplink;?>">
<input type="hidden" name="currency_code" value="USD">
<input type="image" src="https://www.paypal.com/en_US/i/btn/btn_donateCC_LG.gif" border="0" name="submit" alt="Make payments with PayPal">
</form>
</div>
<img style='width: 100%; height: auto;' src='<?php echo WP_PLUGIN_URL .'/'.str_replace( basename( __FILE__ ),"",plugin_basename( __FILE__ ) );?>/images/plugin-logo.png' />
<div style='clear:both'></div>
</div>

<div class="postbox-container" style="width:60%; margin-right: 5%;">

<form action='options-general.php?page=thisismyurl_easy_pinterest.php' method='POST'>
<input type="hidden" name="easy_pinterest_update" value="1">
<div class="metabox-holder">
<div id="normal-sortables" class="meta-box-sortables">

<div id="easy_pinterestsettings" class="postbox">
<div class="handlediv" title="Click to toggle">
</div>
<h3 class="hndle"><span><?php _e( 'Easy Pinterest Settings','easy_pinterest' ) ?></span></h3>
<div class="inside">
<p><label><input name="easy_pinterest_block" type="checkbox" value="1" <?php if ( $options->easy_pinterest_block == '1' ) { echo "checked";}?> /> <?php _e( 'Block Pinterest from this domain','easy_pinterest' ) ?></label></p>

<p><label><input name="easy_pinterest_thankyou" type="checkbox" value="1" <?php if ( $options->easy_pinterest_thankyou == '1' ) { echo "checked";}?> /> <?php _e( 'Include a link to thisismyurl.com discreetly under the widget.','easy_pinterest' ) ?></label></p>
</div>
</div>

<input style='margin-bottom: 20px;' type="submit" class="button-primary" value="<?php _e( 'Save Changes','easy_pinterest' ) ?>" />
</div>
</div>

</form>
</div>

</div>
</div>
<?php
}

function thisismyurl_easy_pinterest_block() {

// look to see if the block option is active and if it is, add a meta tag to the head of the HTML page

$options = get_option( 'thisismyurl_easy_pinterest' );
$options_block = $options->easy_pinterest_block;
if ( $options_block == "1") echo '<meta name="pinterest" content="nopin" />';

}
add_action('wp_head', 'thisismyurl_easy_pinterest_block');

?>