Why can't my Mac Garageband hear my mando through my Steinberg interface?

Why can't my Mac Garageband hear my mando through my Steinberg interface? I have set my system preferences so that Input is the interface and Output is the headphone jack. But all I am picking up is my built in mic on the Mac....the interface is not coming through even though I see that the software knows it's there. Cheers, David