Channels 1/2 from PC go to the Monitor Channels.
Channels 3/4 from the PC go to inputs 9/10 on the console.
Channels 5/6 from the PC go to inputs 11/12 on the console.
Please have a program playing back 5.1 audio then select 5.1 in the control room. Finally press the downmix button. This will collapse the 5.1 mix to stereo.