Found this page from Apple on the 1st gen Apple TV - http://support.apple.com/kb/TS1453?viewlocale=en_US&locale=en_US
One of the things it talks about is making sure you have them set to be looked for, "Choose iTunes > Preferences (Mac) or Edit > Preferences (Windows), click Apple TV, and make sure "Look for Apple TVs" is selected."
Also, do you see the "Devices" section at all in the sidebar? If you do, check to see if it was ticked to "hide" everything in that list by hovering over the "Devices" name in the sidebar.
Lastly, though it shouldnt matter for a 1st gen ATV, do you have the latest version of iTunes?