Aha! Now I see. To add additional compression, you need to run the stereo output of the Fostex through the compressor to its destination device. This will of course compress the entire mix. Or you may install the compressor in the "Insert Loop", which will effect only the signal from that mixer channel.ie: if the bass is recorded on track 2,and you play it back through mixer channel 2, and the compreesor is installed on channel 2 insert, the signal is routed out of the insert jack to the compressor,out of the compressor, and back to the insert jack, where the compressed signal is then routed back into mixer channel 2,and then to the stereo outputs. Just to be sure you're aware, the insert jack is an input and an output.
Sorry if you already knew all this, but it should work this way if you've hooked it up correctly. Twist