Never mind. I figured it out, it required use of some brain cells. So I will try and answer my own question.
The radiator cover which is plastic by the way, is poorly designed. It has these clips that point outward on the ends (opposite directions), but Im not sure if its due to time as it doesn't securely hold up the cover because they don't push outward very strongly and easily comes off on its own.
For 328 xi, to get to radiator drain plug and heat exchanger, you have to take off a solid metal plate (possibly under engine) with 5 #16 bolts. Then take 16 or so #6 bolts off the plastic splash cover.
Right above the plastic cover is the radiator underside cover (plastic). This slides in to the back of the bumper and also clips into some other plastic that hangs vertically (it has the grooves for the clips to fit into, but it does not securely keep the clips in. The cover also has a few screw holes to secure the plastic covering.