There should be deck pumpout fitting marked "waste" (embossed on the flange). It should look exactly like the fuel and water fill fittings. But first you need to find the tank. The pumpout line to the deck comes off of it, most likely at the bottom of a vertical tank wall, but could be on the top with a pickup hose inside the tank. Once you've found the tank, it shouldn't be that hard to follow the hose from it back to the toilet. There should also be a hose from the toilet connected to the tank at or on the top of it and a smaller vent line coming off the the tank at or on the top.
If you can't find the tank, I suggest you contact the previous to owner to find out if there is one on the boat. It may have been removed.
--Peggie