I haven't read the manual, but I'd imagine those inserts are on the input side - after the pre but before the ADC. Don't think you can use them to process signals coming out of the computer without first looping from the outs to the ins, which is what we're trying to do anyway, so...
You want the 1/4" holes furthest left - Playback Outputs B
You should be able to go into those inserts as you've drawn, but you probably shouldn't use a balanced cable for that run. It ultimately doesn't matter whether you go to inputs 1 and 2, or 3 and 4, or really any pair you want.