Garageband and Digitech GNX4 pedal

I'm new to garageband. I have hooked my GNX4 guitar pedal into my macbook via a USB cable. The garageband recognizes my pedal, because I can import mp3 files from the 8 track recorder on the pedal, but I am having a problem getting garageband to recognize "live instruments." I think it may have something to do with some settings, but I can't figure it out for the life of me. Should garageband recognize a live track (guitar for example) through my GNX4 via the USB cable or do I have to input it into my mac in a different way? Ultimately, I would love to be able to have a drum track playing on garageband, playing my guitar through the pedal and recording it live into garageband, while also being able to hear the guitar on my amp. right now the pedal out goes into my amp and like I said the USB goes from my pedal to my Mac. I have a feeling the USB is only recognizing the mp3 player on the pedal and not the instruments. HELP!!!!!!!!!!!!!

Thanks!!!!!!!!