I/O Setup Question

[mtwhite]

I'm having trouble getting the playback/monitor settings to work for me.

Here is my setup:

Instrument>Behringer 12 Channel USB mixer>iMac>Pro Tools.

I've pulled up an existing project with a drum loop.

I have tried Pro Tools Aggregate I/O and USB Codec and neither will allow me to hear any track. I have tried headphones into the Behringer and into the iMac. I can hear other programs through the iMac (iTunes for example) and I can hear the instrument live through the Behringer. I know that part is working.

With the above configuration, how should my I/O, Hardware and Playback Engine be set?

Thanks for any help.