The 'plug' on the top of the axle is a vent. The plug on the side of the cover is the fill/level plug. If there is not a plug on the bottom of the cover then it'll need to be removed for draining.
To seal the cover, use new gasket, if old gasket can be removed and reused just use a good sealer on it. Make a new gasket. Or use a 'Form -A- gasket' compound.