[OLDKID]

The drain plug should be evident . On most 150's the drain and fill plugs on the CVT are 10 mm head similar to the bolts that hold the side cover on , but with a thicker sealing washer . And the location usually has a thicker boss . Who knows , maybe you have to measure and fill through the vent ?
I recently bought a lawn mower with a Briggs & Stratton engine and ........
NO DRAIN PLUG , GOTTA FLIP IT UPSIDE DOWN TO DRAIN ...... As far as your exploded view , that's only 1/2 of the case the drain plug location might be on the other case half .