If you create a software track, as Hangtime suggested, say, the default piano, and play on your axiom, do you see the meters lighting up on the left of the piano track? If you do, at least you know that Garageband is seeing your axiom. If you are seeing the meters light up and also hearing the sound, you're in business. The next step would be to click open the info pane on the right, and select any other software instrument your heart desires.
If you are seeing the meters light up on the piano track but not hearing anything, that would be another problem to troubleshoot, but at least you know GB is seeing your controller.
