Hello Cello the good news is that the Oxygen49 even with OSX Mountain Lion works with Garageband 11.
Assuming you have the 3rd generation (look under your keyboard on the back, it should tell you).
First, you'll need to close out Garageband. Go on Axiom's site and download the DIRECTLINK software. Take the read me file and print it out. Then install the driver (instructions will walk you through).
Restart Garageband (You may need to do this a few times or reboot your machine if you'd like).
Click the SELECT button then cycle till you see a P number P1, P2, etc, You want to select P10, that's the preset for GARAGEBAND. When you select the P or PRESETS, you'll hit the plus button to the right of it, until you find P10, that's it, there's no ENTER button to push like you would think, it just stays on P10, confirm that just by turning off your keyboard and turning it on again you'll see you're still on preset p10 (unless you again hit the plus or minus button).
Once you're in Garageband again, let's test it out. Select about 5 tracks, open up 5 software tracks. Maybe just make some music on them. Now look and see you'll be able to control each individual volume on each track with the slider (sliders 1-5), slider 9 is the volume slider, you have 16 sliders to use (1-8, then 8-16), with the last slider (odd man out) being the master. To mute a track, simply hold the C26 button (under last slider to the right) and select the button that relates to that track. (ie, Button 1, track 1, Button 2, track 2, etc)
Best feature.. the transport buttons, loop track, reverse, forward, stop, play and record! I'll be honest, the nobs I'm still figuring out, that's why you'll need to view the "read me" file. One controls the reverb, the other controls the visual equalizer, but I imagine there's a way to program them to suit you. I would be safe to say that in the entire internet at least in my searches there has been no clear instruction on how to enter midi data on the keyboard that is simple and easy to understand.
But I hope this helps you and others looking to use the Oxygen49.