'selected device has no input controls' - this is because it doesn't need them - it's for interfaces which have no hardware level control: it doesn't affect whether it works or not, it's just that there is nothing to adjust.
Presumably the mixer has a main audio output: do you get audio out of this?
If no: then you haven't routed something correctly in the mixer.
If yes:
If you go into System Preferences>Sound, choose 'Input': can you see the mixer in the list?
If no: driver not properly installed
If yes:
Select the mixer as the input (this is for testing, it doesn't affect GB).
If you feed the mic into the mixer and you can hear it on the main audio output, do you see the meter in the Control Panel moving?
If no: the interface between the mixer and the computer needs looking at.
If yes:
In Garageband, check you have the mixer set as the input.
Record something on a track and play it back (having checked that playback of anything else, e.g. a loop. is OK): can you hear it?
If yes: enable monitoring in the track info.
If no: then we need to look more closely at the problem - please report back.
