hi, I have the same issue. I have a Presonus USB audio interface. Working perfectly with Garageband. You can see signal on the Mac audio preferences. But nothing comes through BIAB. No signal on the meters and no audio recorded.
I've done as Andrew suggested above …
Have you?
in Band-in-a-Box, go to Preferences | Audio Settings, and make sure you are seeing what you expect for the input.
What are you seeing? There should be a number fo choices—you need to select the Presonus as your input device.
In the bad old days of the Mac OS, you had to select your interface for system audio, then select it again for your app or DAW. This is no longer true. You now select your interface directly from within the app settings.
It must be present in System Audio, of course, or that indicates it's not turned on but you don't have to select it there.
This flexibility lets me track in my DAW through my interface but audition tracks in iTunes through my speakers if I want. It also lets me make a simple setting change and now I'm mixing through those same speakers as I prefer. You didn't used to be able to do that.
The beauty of GarageBand is that Apple takes away a lot of your choices — that is also the down side of GB. Some of my singers are using Presonus interfaces with GB. This makes more work for me and I intend to wean them to, really, any other DAW. If they buy Logic, the interface is the same but those limitations are gone. There are other free DAWs for the Mac, however, if the $199.99 is an issue. Apple does have a bargain: That same money in the Education Store gets you the Pro apps Bundle with Logic, Final Cut Pro, Main Stage and two other video apps.