Help wit activating iTunes account on Apple TV?

Are you trying to setup iTunes Home Sharing on your Apple TV?
If so, here is a troubleshooting guide provided by Apple to help you in your efforts:
Troubleshooting Home Sharing

If you're trying to do something else, please let us know and we'll try and help you figure out the issue between iTunes and the Apple TV box. Oh, what generation/version of Apple TV are you running?