The most obvious question: have you created and selected a software instrument track?
In GB's Audio/Midi prefs, does it say "1 midi input detected"? (You don't have to select a midi device, GB listens to all incoming midi signals.)
If yes: Is a little blue dot in the "LED" window lighting up each time you press a key on your keyboard? That would mean that the midi signals are coming through.
I came across this discussion today since I have the same problem.
For me, in the Prefs, it DOES say "1 midi input detected" but I'm unsure what "LED" window you're referring to. Best I can tell, nothing happens when I press a key on my keyboard (I'm assuming you mean the musical keyboard and not my Mac keyboard).
I having this problem as I am trying to learn to play the piano. I'm using the lessons where you play along, but GB isn't seeing my keyboard connected.
By LED window, I mean the counter that counts bars or seconds and looks like a little LED display. It has a little dot in its lower right corner (I think) that should light up whenever it receives a signal from a keyboard.
If a midi device is detected but no signals are arriving at the computer, it makes sense to check whether the keyboard is sending anything in the first place.
This answer comes a bit late I agree (...) but as I'm about to try using my old Keystation 49 with Sibelius, I've found your post about you having trouble plugging the same keyboard to your mac.
Try this link if you still have your Midiman.
2 hours ago, I've had the same problem as you (2 years ago...) but it seems to be fixed now (I can hear the different instruments in GB despite the fact that I can't see the Keystation 49 anywhere in the settings (nor in the USB devices or the Audio MIDI Setup).
Talk to you in 2014!
I seem to have a similar problem. I am trying to connect Studiologic SL 990 to my Mac Mini (Mac OS 10.7.3). The computer "sees" the keybord , which is connected throu CME U2 Midi cable. But no sound comes out of Garage Band. When I connect the keybord exactly the same way to my old white iMac, it works ok. Would anyone have an idea how to fix this?