No Sound From PreSonus Audiobox USB

Hey, I'm trying to use a PreSonus Audiobox USB interface with Garageband 09 to record vox and guitar. GarageBand recognizes it as an available input, and I have the volume all the way up both in the program and on the interface, and still have no sound coming through, either on the monitor or actually being recorded. Sometimes the clip light registers on the interface, so i think that's getting sound. But nothing comes through my computer.

I've tested all the mic, mic cable, guitar, and guitar cable, so it's nothing obvious like that.

Am I missing something? Thanks.