Looking at the M-Audio site I see that the only operating system it supports for mac is:
Mac OS X 10.4.11 or 10.5.6 If you are trying to connect via USB you won't have the drivers to do it. I have a older Edirol PCR-300 that I can't use either via usb. Going to have to dig up the power supply and connect it via midi cables in/out.