There is a round plug with a big hex hole in the middle on the side of the gearbox facing the bumper.
Just to be clear did you mean top up or replace? The gearbox oil shouldn't have to be topped up, only replaced. Unlike engine oil, it doesn't go anywhere so you should have the same amount as it had since new.
If you are replacing, make sure you can undo the top plug before draining from the bottom plug. The last thing you want to do is drain the oil and then find you have issues undoing the top plug.