Assigning Hardware Controls in MainStage.

Does anyone have any idea about being able to control the modulation wheel with an expression pedal? I have a M-Audio 88es and a Motif 8 Classic and I can't get the pedal to control anything but volume. When plugged in, the expression pedal overrides the volume control on my 88es, and all attempts to map the parameters in Layout have ben fruitless. I would prefer to use my left hand to adjust volume, and have the ability to modulate the sound with my foot.

M-Audio 88es, M-Audio Firewire 410, MacBook Pro, Mainstage 1.0.2 283.7, OSX 10.5.2

Thanks!