During a recent rehearsal with my band, System 9, I dragged out my PowerBook to show the boys the wonders of GarageBand. Our bass player was interested in plugging into the program to see how the effects fared and we happened to have a Digidesign Mbox on hand. Yet, try as we might, we couldn't make GarageBand see the Digi box. It appeared as expected in the Audio MIDI Setup utility and the Sound system preference, but was a no-show in GarageBand.
Demonstrating the power and glory that is the Macworld Forums, I stumbled upon a thread where user vf124 commented that he'd successfully connected GarageBand to his Mbox.
For those who find clicking on the link above too inconvenient, here's the answer:
To make any Digi device available to GarageBand, you must fire up the Digidesign CoreAudio Setup application, click the Supported Applications button in the resulting Digidesign CoreAudio Setup window, click the Add New Application button in the window that appears, navigate to the GarageBand application, and click Choose. Your Digi gear will now be available from the Audio/MIDI tab within GarageBand's Preferences window.