Problem
The Apple TV no longer appears in the iTunes menu.
Solution
There are a number of reasons why this may occur, but fixes for most cases can be seen in the guide at the link below.
http://support.apple.com/kb/TS1453
Notes for VirusBarrier X6 users
If you use Intego VirusBarrier X6, your connection to Apple TV may be blocked (usually because VB previously detected a "flood" attack involving ATV). This means Apple TV will not appear under Devices in iTunes. (Historically, Intego software has been very unobtrusive, so this is unusual.) The solution is to update VirusBarrier X6 to the latest version, then click on the Antivandal Tab, then the 'Trusted Addresses' tab. Finally, click on 'Trust Apple TV' checkbox near the bottom of the application window. You may need to restart iTunes.
Unfortunately, at this time, this solution may also fail. In that case, follow these steps to manually "trust" Apple TV:
First, revert VirusBarrier X6 to it's default settings:
-Select 'VirusBarrier X6' in the top-left menu bar of your screen.
-Choose Preferences from the drop down menu
-Click the Advanced icon in the new window that appears.
-Now click 'Revert to Default' and enter your admin user password.
Now, follow these steps to allow access to the Apple TV:
To locate your Apple TV's IP address:
-Go to the Settings menu on Apple TV
-Select General
-Select Network
Now, make note of your Apple TV's IP address.
Now, open your VirusBarrier X6 software:
-Select the AntiVandal icon
-Click on the 'Trusted Addresses' tab
-Click the small '+' sign in the lower left corner of the application window
(you may need to click the small padlock icon to allow changes)
-Enter the IP address from your Apple TV
-Click 'Okay'
Test to see if you're able to connect to your Apple TV.