Uhm, just click the "Sync" button under Apple TV in iTunes.
Ok i'm feeling quite stupid but where did the sync 'button' go? I only have two tabs available when clicking on appletv device. I have summary and photos. I've selected new photo folders to sync and I only have a cancel and apply button. I don't even have a 'movies' tab.
You're iTunes isn't the default sync device for the Apple TV.
There's only one computer that can be completely synced with it at once.
go to the Apple TV menu and select your computer as the sync.A number will pop up on your iTunes interface. Type in the numbers and off it goes.
You can then select what is synced and what isn't from your iTunes .