There is no FTP access to free hosted WordPress.COM blogs and we bloggers cannot install individual plugins or third party themes into them. Those found on the internet are for WordPress.ORG installs which run on different software.
On free hosted wordpress.COM blogs we can only use the themes which have been coded to run on this multi-user software and made available to us here. > Appearance > Themes and no others. We cannot edit either themes or templates on free hosted wordpress.COM blogs. > http://en.support.wordpress.com/themes/editing-themes/
There is no upgrade you can purchase that changes this. If you wish to have complete control over a blog, the ability to upload themes and plugins, and to edit themes and templates then your option is to hire a web host and get a free software install from WordPress.ORG and self host it.