The fill plug is about halfway up the diff cover. The fluid quantity required is no where near the capacity of the diff housing, just like a manual transmission. The fluid clings to the gears, so it doesn't need to be entirely submerged.

The fill plug is usually a large square head with pipe threads. The drain plug, as you've found, is on the bottom facing down. Use a sealer on the threads and tighten to the point of it being snug. Pipe threads don't require a lot of torque to seal. I like to use Permatex teflon paste on pipe threads.
--
forums.turbobricks.com