Currently Being ModeratedDec 16, 2012 7:55 PM (in response to jamerz3294)
Sounds like the soundboard has a usb output, depending on the brand you may need to download some drivers. Then in garageband you will need to choose the soundboard in GarageBand > Preferences > audio midi and choose it for input. Start a real instrument stereo track and you're ready to record.
This is a little over simplified but you can find tutorials under the help menu in garageband or on youtube.