FastTrack Pro USB Audio/Midi Interface: no signal

[prosodylab]

Hi,
I recently installed the FastTrack Pro 4x4 Mobile USB Audio/MIDI Interface with Preamps. I'm running on a Mac OSX v. 10.5.8 with a 2.8GHz Quad-core Intel Xeon processor. I've installed the drivers on my computer as well as the ProTools software, have restarted the computer, and have confirmed that the device is connected in the M-Audio system preferences. I have Shure WH30 Condenser headset microphones that connect to the mic/inst inputs on the M-Audio device.
The device is connected and the power is on (both the power and the 48V lights on the front panel are illuminated), but the signal light to the microphone input channel is not. When I plug the microphone the signal light and the clip light flicker but do not stay lit, even when I adjust the gain knob and the the microphone preamplifier connection. When I select this device in my system preferences sound settings, it shows up as a device but does not show a sound signal. I've tried two difference microphones, and the other MIDI interface (I have 2), but with the same results each time.

Any advice would be appreciated!
Thanks in advance :)