I'm sure you've resolved this by now, but I'll post this just so that others don't panic as well…
I bought the M-Audio expression pedal as well and had the exact same problem. I fiddled around with the switch at the bottom (it should be in the 'M-Audio' position), rocked back and forth, and messed around with the potentiometer… this actually made things worse, so I simply unplugged the power from the TF, plugged it back in and that fixed everything. Rebooting is all it takes! I was then able to reassign values and sweep from 0 to 100.
