Read > http://en.support.wordpress.com/stats/#additional-info
There is no FTP access and no blogger installed plugin capability on free hosted WordPress.com blogs.
See here for a full comparison WordPress.com vs. WordPress.org
There is no upgrade you can purchase here that provides FTP access and the ability to install plugins and or any third party themes.
If you want FTP access and to install plugins or third party themes into a site then you will have to hire a web host http://wordpress.org/hosting/ and set up your own WordPress.org install.
NOTE: WordPress.com and WordPress.org have different log-ins and run different versions of themes with same names. 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.