Do you mean from the joint between the two halves of case? Quite simple to fix, just remove the driveshaft flange cups by undoing the two Allen head bolts in the centres of them. Then remove the ten or so 10mm bolts from all around the casing (do not remove the torx head bolts on the front of the box!!!) and tap the clutch housing side off. Stand the box on it's end while you do this with the black end cover down. Then clean both mating surfaces and scrape with a blade to get anything off them, apply some sealer and put the two halves back together, torque is only something like 20 lb/ft but i just normally nip them up by hand to what feels right to me!
The best thing I've found as a sealant is something called Threebond 1194. People use this for sealing porsche crank case halves together and it seems to do a good job, although it's not particularly cheap. I also put a bit of locktite on those Allen bolts in the driveshaft flange cups to keep them tight as you don't want them coming off.
