For the level there is a screw facing the outside of the sled on the cover a couple inches up from the bottom. Add until oil slowly comes out of this hole. Gear oil API GL-3 SAE 75 or 80 is the factory oil spec.
Also check your chain tension with the long bolt stickiing out the front of the case. Loosen the locknut at the case, turn the bolt in finger tight. If the bolt is already tight, loosen it up a little and then run it it finger tight, you should be very close to the initial setting then, and it helps you get a feel for it.
What you should really do is drain the oil, remove the cover, and disassemble-clean-check the complete chaincase and bearings. Keep track of how all the parts come out so you can put them in the same way. The oil can be drained by removing the bottom most cover bolt, just try to direct the oil out of the bellypan of the sled and be ready with rags to not make too much of a mess. Also be aware that the cover gaskets tear often on these, so you may want to order one from your dealer, or you can make your own if you have the material, and I use a thin coating of sealer on at least one side, and if there are any questionable areas.