Please check that you have the latest version of iTunes installed.
Then check that you have the latest version of Apple TV software installed.
If you're using Mac OS X v10.6 (Snow Leopard), make sure that you have Bonjour Update 2010-001 or later installed. (You need to restart the computer after installing this update for it to take effect.)
Verify Apple TV is powered and properly connected to your television and you can navigate the Apple TV interface.
Verify Apple TV is properly connected to your network and the network is properly configured. When configuring Apple TV, if you do not see your wireless network, exit the selection screen by pressing the Menu button then go back in and the software will rescan.
ou can't stream and sync from different computers at the same time. If you have configured Apple TV to stream from another iTunes library, Apple TV won't be available in iTunes. Go to the Sources menu on Apple TV and select the computer you're trying to sync from.
Verify that the computer you are using can access the iTunes Store.
If you're using Apple TV in a school or office environment, or other network setup , please make sure Apple TV is on the same network segment as your computer or else iTunes won't see Apple TV.
If Apple TV still doesn't appear in the iTunes source list (below Devices) after you verify the above items, try these steps. After attempting each step, check the iTunes source list to see if Apple TV appears. When Apple TV appears in the iTunes source list, you can stop troubleshooting and continue configuring Apple TV.
1. Restart iTunes (quit or exit and then reopen iTunes).
2. Reconnect Apple TV to iTunes.
If you are still having trouble, you can try to reset your equipment by unplugging power cords for the following:
Wait thirty seconds and then plug everything back in again.
4. Restart your computer.
5. Rejoin the network (wireless) or restart your network router.
Configure firewall software.
If you have Mac OS X 10.5 (Leopard), read this article for information on configuring Firewall. For Mac OS X 10.4 and earlier, make sure UDP port 5353 (Bonjour) is open on your firewall. If you use Windows Firewall, follow these steps. Otherwise, if you use other firewall software or hardware, check the software documentation on how to open UDP port 5353.
Please provide all your Mac info , are you fully updated , please use Apple menu , software update . to check .
for updates , you mac info can be found in Apple menu , about this mac .
I have had this issue myself , while connected to a wireless router , it does depend of the setup. Make sure the router setup or firewall allows for bonjour to work , the Mac OSX firewall in 10.6 is no problem if updated .