ive never drained the oil on one.
but the filler to top up is on the top of the gearbox , right hand side front, our 6711 has the old duncan floorless cab, so the filler is sort of next to the brake pedals, asuming yours has a floor there should be a cover plate, about 2 inches diameter , somewhere near your right foot, remove that and you should see the filler plug under it.
the one on our 4712 has the return pipe for the hedgetrimmer spool valve screwed into it, instead of a bung.