Shows where on your account ? If you mean in the 'manage devices' section of your account then computers and/or devices only show there when you've used iTunes Match or automatic downloads on them, or redownloaded past purchases : http://support.apple.com/kb/HT4627
AppleTV is not synced with your iTunes library so there is no reason for it to be listed in the Devices list of iTunes. iTunes does not manage your AppleTV. The AppleTV has no storage for music and video, just a buffer to play music and video.
AppleTV functions all work as designed without it being listed in iTunes, as long as both your iMac and AppleTV are logged in with your AppleID and Home Sharing is enabled in itunes and the AppleTV.
Message was edited by: bobwild