It should appear all the time and its appearance is automagic. There's no settings under your control to make it appear/disappear.
Devices in iTunes are mounted, like drives are in the Finder.
If your AppleTV doesn't automatically appear, quit iTunes and restart it. The AppleTV and iTunes have been known to drop their connection. They don't do a good job of reestablishing this connection once it's lost, so restarting iTunes will often find the AppleTV.