Hi Claude,
I was looking for the same answer - I just got my (used) mpk25 and it works great, since I play on the keys and pads. However, couldn't figure out how to make stop/play and rewind back/forward buttons work, they just simply won't do anything.
Mpk works great on fl and Ableton. So, I guess something is wrong with GarageBand setup.
As I see this is the quite old post, so didi u figure anything out?
Your help would be appriciated!