[RWP] setting a USB input?

[colin McDonald]

Hey guys:
I'm wondering if anyone can help with setting up reper to listen to a USB 
input?
I just bought a fender mustang IV with USB direct out capability.
I went into devices in preferences and set the record to USB...but I can't 
seem to get it to let me go in and enable that input even though it's 
selected.
I know it is no longer using the microphone or line in input on the 
soundcard, but nothing seems to be coming from the USB port either.
The amp automatically recognizes when the USB cable is connected, so I don't 
have to go in and set it to USB output or anything.
Just wondering if I'm missing something?

Regards
Colin