Instructions
Connect the USB cable on your headset to any of the USB ports on your PlayStation 3. Use the controller to navigate to the "Settings" menu on the XMB dashboard.
Scroll down through the "Settings" menu until you reach "Accessory Settings." Press "X" to access the sub-menu, then scroll down to "Audio Device Settings" and press "X" again.
Scroll down to the "Input Device" option and press "X," then highlight your headset's name--it should be the brand and model number or something similar--and press "X" to set it as your input device. Scroll down to "Output Device" and use the same process to set your headset as the output device. Alternately, you can leave this as the default so you can still hear sound from the TV.
Set your mic level by highlighting the "Microphone Level" option and setting a numeric value -- the default is 3 and should be sufficient for most models.