The center rubber plug can be pulled out with your fingers. It is just pushed in, not threaded. Try rotating it first and you will see that it is just a simple rubber plug with a lip to catch on the plastic.
Once you remove that plug, fill the oil until it starts to run out. put the plug back in and you are good to go. I drain and fill mine every two years and have not had to replace a rubber plug yet. The reason why you do not fill past the lip of the rubber plug is because centrifugal force would just force the oil out and probably blow the plug out with it since it is just a simple push fit.

