Q: Can your harddrive be played on your apple T.V

Not directly, however, you can play items off of a hard drive via home sharing.  Anything in your itunes library on a computer that is on the same network as apple tv can use home sharing to be played on apple tv.  See

http://support.apple.com/kb/HT4352

Windows or mac?

Have you tried these?

• Drag a file or folder from the desktop to the iTunes window.If you add a folder, all the files it contains are added to your library.
• In iTunes, choose File > “Add to Library,” locate a file or folder, and then click Choose.

What kind of file sare you trying to add?  Could it be that the files you're attempting to add are not compatible with itunes?  Below are what types of video are compatible with itunes.

• Video content purchased from the iTunes Store.
• QuickTime and MPEG-4 movie files that end in ".mov", ".m4v", or ".mp4" and are playable in QuickTime Player.
• Video podcasts.
• iTunes Digital Copies.
• iTunes Store Movie Rentals (Requires iTunes 9 or later).

Handbrake is one of the most widely recommended format converters.  http://handbrake.fr/downloads.php

You're welcome