For the 900 you will need to use the Zune software for loading music to the device. If you have MP3's that you play in Itunes those will load in Zune without a problem. If they are songs that you bought from Itunes, you will have to burn them to cd's and then rip them back as mp3's onto your computer or use a program like Sound Taxi to convert the itunes music files to mp3's.
With the 920, we will probably be using the Xbox music app to load music. Not 100% though, someone else may correct me on this one.
I hope this helps!