The only themes we can use on free hosted WordPress.com blogs are found here > Dashboard > Appearance > Themes
If you purchased a premium theme from WordPress.COM see here > http://en.support.wordpress.com/themes/premium-themes/
There is no FTP access to free hosted WordPress.COM blogs and we cannot install plugins, upload third-party themes or edit templates or themes. Those found on the internet are for WordPress.ORG installs which run on different software.
There is no upgrade that you can purchase from WordPress.com that provides FTP access and the ability to upload third themes. To use any theme other than the over 200 found here Dashboard > Appearance > themes you will need to hire a web host and set up a self hosted WordPress.org install. http://support.wordpress.com/com-vs-org/
If you are referring to a WordPress.ORG install that you bought a theme for then you are posting to the wrong support forum. This is the correct forum for your software http://wordpress.ORG/support/ If you have not registered a WordPress.org username account yet you can do so on the top right hand corner of the page I just linked to so you can post to those support forums.
WordPress.com vs WordPress.org: The Differences