When I had my old E46 I was certain that not a single person had ever opened that plug in the cars life. When I did it, it just turned with no problems.
Easiest way to do a flush is a garden hose trick, but it'll require you to remove your thermostat. IF you wish to do so, you need to drain the rad, then remove thermostat and stick a garden hose in the opening and wait for all the blue water to come out. Its the easiest way to do a flush, but its not really suggested as you need to remove the thermo to do so.
If you do coolant flushes on a regular basis, you can attach a water line to the block as showed on the pictures below. That way engine flushes are easy, quick and you dont get water spills all over the place.