WordPress.com is a multiuser blogging platform. Our blogs are not free standing. There is no FTP access to free hosted WordPress.com blogs and we bloggers cannot install any plugins. There is no upgrade you can purchase that will provide FTP access and the ability to install plugins, upload themes, and/or edit templates and themes in a free hosted WordPress.com blog.
To use plugins you must hire a web host and then set up your own WordPress.org install.
That's not a blog being hosted by WordPress.COM and you are posting to the wrong support forum. WordPress.com and WordPress.org are completely separate, have different log-ins and run different versions of themes with same names. http://support.wordpress.com/com-vs-org/
We provide support only for free hosted WordPress.COM blogs and not for WordPress.org software installs. If you don't have a username account at WordPress.ORG click http://wordpress.org/support/ and register one on the top right hand corner of the page that opens, so you can post to the support forums there and receive advice from WordPress.ORG bloggers.