The LSD part of the gearbox is a viscous coupling that is connected to a cv shaft that is input into the diff which is the same as any open wheeler gearbox.
the leak can either be caused by the viscous coupling which is not repareable you will have to order a new coupling.
or from the gearbox output shaft seal which sits under the LSD unit in the place of your regular axle seal on a open diff box.
part number for the output shaft seal is 90311-50015 (i have one sitting right in front of me as it so happends.
the seal you see on the outside of the LSD casing is just a dust seal.
cheers
ryan