Pop the normal L & R i/o across the stereo gtr group.
Then feed the kick signal into one of the TWO extra sidechain input sockets on the left channel.. (TRY EACH ONE)
Have the middle STEREO button engauged
Then mess around with the controls on the front untill you get the compressor to work on both sides triggered by the kick signal.
dont for get to try my half out jack plug trick above - ALSO try using a STEREO jack going in with the trigger..
keep at it... you will get there..