I spent lots of time trying to fiddle around with getting this to work, and the answer was relatively simple:
If you can't homeshare on itunes between Computers or AppleTV, try turning multicast on
1) login to router
2) Go to Advanced tab
3) Select Advanced Networking on Left hand column
4) Select checkbox to enable multicast streams
5) Save the changes
Note -- If you have a network extender:
In order to get all my machines working, I also had to enable multicast streams on my network extender as well. The IP address for the network extender is 192.168.0.50 or type in DLINKAP in the URL bar and follow the relevant instructions for your model number.
I hope this helps saves hours of frustration for as many people as possible. AppleTV and iTunes sharing works great now.
Please note if you use a software firewall on your PC or Mac, you may need to also free up TCP port 3689 and UDP port 5353. However there are plenty of support articles at Apple / Norton / Microsoft / etc... on how to do this for your relevant firewall....good luck!