on my pre-facelift 98 k11, the drain plug is where yom mentioned but i filled mine up by first disconnecting the speedo cable from the gearbox (cable leads from the dashboard down to the top rear of the gearbox.
undo bolt thats holding the speedo pinion onto the gearbox and remove the pinion. now ur left with an access hole into the gearbox to fill up.
my cylinder head was off at the time which made filling easy but might be a pain to access with an assembled engine bay cos of airbox, inlet manifold, etc
or use a hose with a funnel