I had this same problem as well. I figured out that Bonjour Service was disabled on my computer and Itunes seems to require to service to shared your Libary over your local network.
After enableing this service I was able to see my computer library on the apple tv.
To check to see if the Bonjour Service is disble go to Control Panel/administrative Tools/Services.
Look for Bonjour Service, if its disbale or not started. Enable it if it is disabled or start it if its not started already.
Hope this helps.
Cheers for now.
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