USB headset and MBP

I'm not sure this is the best place to post this question. If there is a better forum, please let me know.

I spend a lot of my time in the late evening hours in a flash-based internet chat room where we broadcast audio and video. I do fine if I use my internal speaker and my iPod earbuds.

I would like to use a headset that has the audio transmitted to the earpieces (which works fine) along with an integrated microphone (which isn't working).

I've tried a couple of different headsets whose microphone input don't work. A technical paper I read said that the input needs to be powered/amplified. This would make sense, because the headsets I've had were straight from the headset to the audio input port without amplification.

However....

I have a Plantronics DSP-400 headset that plugs into a USB port. I get beautiful audio coming to the headset speakers, but I can't get the microphone to work.

I went into System Prefernces > Sound and the Plantronics doesn't show up as an option for the Input subtab. Naturally, I can't get any sound from the headset.

Should this be a Plantronics question?

The Plantronics site for this model says that there should be no drivers for Mac OS X, that it should work as is.

Thanks!
Deb