Tascam US-1641 with Garageband

I have a Tascam US-1641 recording interface that I want to use with Garageband. I set up the input and output to go through the interface, and I created new tracks and set them to different channels in the interface. I then activated them and hit record. As it recorded the red highlight along each activated track moved, but there were no sound waves and the meter's in Garageband showed no sound. Once I stopped the recording the red highlights disappeared and it was as if I had done nothing.
The gain knobs on the interface were turned up and changed, and it did nothing. I read somewhere that I have to set my Macbook's system preferences to read the interface as input in and output in the sound category. The problem with that is it doesn't appear there...it only appears in Garageband.
Does anyone know how I can get the interface and Garageband to work together?