On WordPress.COM, you cannot use themes from any place except the ones available there. And there is no FTP access either. These forums here are for self-hosted WordPress.ORG sites - not the same as WordPress.COM - see this for more about the differences:
For .COM support, you need to go here:
http://en.forums.wordpress.com/ (but there is no way to use your own theme there)
If you want to use another theme, you can always pay for hosting and use the free WordPress.ORG software on your own site, and then we can help you with any questions here.