There is no FTP access to free hosted WordPress.COM blogs and we cannot install individual plugins into them Plugins are for wordpress.ORG installs which run on different software.
Which "support site" did you check? The codex is only for WordPress.org bloggers and has no application to us here at WordPress.com.WordPress.org self hosted installs run on different software.
This is the support documentation link for WordPress.com users http://wn.support.wordpress.com At that site the searchbox provides results form the support documents and from forum threads. Type in "FTP" and/or "plugins" and you will the links I found there.
As you are new to WordPress.com I can recommend a comprehensive tutorial. The Learn WordPress.com tutorial is divided into progressive levels and presented as a series: http://learn.wordpress.com/