I would like to be able to play my guitar along with songs I have in iTunes and listen to a mix in my headphones. I have an Apple PowerBook, and an M-Audio M-Track interface, but I don't know how to get them mixed together. I'd rather not have to import a song separately into Garage Band, but rather just play songs in iTunes and be able to play my guitar along with them.
Just set both iTunes and whatever software you are using to monitor your guitar (DAW, plugin host, software mixer, whatever) to use the same sound card (in this case your M-Track) and same outputs (in this case the ones assigned to the headphones).
For iTunes routing you need to set the whole system audio to use your sound card. You can do it in the system preferences or by alt+click the speaker icon at the top of the system bar in some OSX versions (the one that has the time, date, etc).
The other routing will depend on how you are monitoring your guitar. It should be very straight forward, just check the manual of whatever software you are using.