Slight correction to my previous post - in which I was trying to state that with the Apple TV on it’s own - will not see the shared iTunes server - but it will see the iTunes libraries of any PC or Mac that has home sharing enabled provided you have enabled home sharing on the Apple TV device.
If you go to any of those PCs or Macs and open iTunes - you should see the MBL’s shared server listed under Shared. Near the bottom right of the iTunes screen - you should see an upright triangle inside a box - click on this triangle to select the output speakers to use for subsequent playback - you would then select the Apple TV device as the speaker output. Anything you play from this point on from this local iTunes - whether audio or video should now use the display and speakers on the TV connected to the Apple TV - regardless of whether the content is coming from the local iTunes library or from the MBL iTunes server.
This should work for any PC/Mac on the local network with iTunes - but I believe only one local PC/Mac can control the output to the Apple TV device at a time.
You have to initiate the playback from an iTunes enabled PC or Mac - you can’t initiate playback directly from the Apple TV device via the Apple TV’s remote.