To record audio from the X1204USB into your computer:
1.) Connect a USB cable from the X1204USB mixer's USB port to the computer's USB port.
2.) Push down the MAIN MIX button in the source section.
3.) Pull up the MAIN MIX faders to unity gain (-0-) for signal to go to your computer
4.) To listen to playback from the computer, push the red 2-TR/USB down in the source section. 
5.) Turn up the white PHONES/CTRL RM knob to listen in your headphones