I've got my main PC that has movies, music, tv shows and all on its itunes that i've built up over the years. I've just got myself a MacBook Air, and have itunes on it now, but I can't get my movies or music onto it.. any reason why?
A little more detail though, how are you trying to get it onto it?
If you just take the files from your iTunes library (the files themselves, not via any iTunes stuff) and copy them to your Macbook, then add them to the iTunes library on the Macbook you should be fine (iTunes on mac keeps your music organized in it's own folders by default, so you can go ahead and delete the original files you copied over - better check first, it's been a while since I used a mac). That's for all your non-iTunes store stuff. If you bought your stuff from iTunes, just punch in your username/pw and dowonload it again.