There should be on top of either the left or the right side case an oil screw. It either has a slot in it or a finger grip on it. If your just topping it up and it has a sight window usually on the right side case down the bottom or sometimes on the left, it sits between the 2 marks and upper and lower mark.
If your filling it from empty it should be written on the filler how much oil it takes or will have two sets of numbers the higher is when you are replacing the oil filter and the second lower is when your just replacing the oil only.
Best to have a measuring device, most oil bottles do. Only fill it with the correct grade oil and with the correct amount. You should have a hand book it will tell you in there how to do it.