problems ftp-ing custom theme to wordpress blog

help!
i signed up for a free blog using wordpress.com.
i got a username, password and URL ok (looking like: http://myblog.wordpress.com)
Now. I found this software on the net called WordPress Theme Generator. I created my own theme and saved in in a file on my computer, ready to be uploaded.
I also have downloaded an FTP client (Uploader) – don’t understand it – then another one (Ipswitch WS_FTP Home), looks simpler but my real problem is that i don’t know how to find what my FTP server address is! Where do I find it? I’ve had several guesses already, typing several ftp-looking addresses in the FTP client, but no success, I’m almost giving up, it keeps saying the connection can’t be made because the ftp address is wrong! Help please!