Open ableton first then garage band, create an audio track in ableton and set the "audio from" option to garage band and then set the monitor option (it will be three buttons saying "in, off and auto") to in. this will enable you to hear music from garage band. if you want to record hit the arm button at the bottom of the track and push the circle next to any of the clips in the channel. let me know if this works cos if it doesn't i'm stumped.
I'm so thankful for all the help i recieved when i was starting out clueless. And now i want to pass it on. It really is a beautiful online community we producers have.