That's not possible on a free hosted WordPress.com blog. If you want to install plugins of any kind then you will have to hire a web host http://wordpress.org/hosting/ and set up your own WordPress.org install.
There is no FTP access and no plugin capability on free hosted WordPress.com blogs.
There is no upgrade you can purchase here that provides FTP access and the ability to install plugins and or any third party themes.
See here for a full comparison WordPress.com vs. WordPress.org
WordPress.com and WordPress.ORG have different logins. 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.