RE: Changing Oil 2004 Sportster
There's not a drain plug. There is a rubber hose that dead ends into the frame. You loosen the hose clamp and drain the oil from the hose. Make sure you have the filler cap off. Also, Harley suggests that when you fill the tank back up, you let a little of the new oil run out of the drain hose before buttoning it back up. I reckon they are afraid it might get airlocked.
There is a drain plug in the vicinity but it drains the oil from the primary.
I would get a shop manual for your bike. It will save you a bunch of time and money in the future.