There are two ridged tabs on each side of the housing. Use a flat-blade screwdriver to GENTLY pry outward....and probably another screwdriver jammed at the top or side to coax the two sections apart.
The sections will seperate dramatically (as if you broke it). I've had to replace the bulb twice, and the only surface damage is the slight distortion of the mated section where I twisted the screwdriver at the top to get the sections to seperate.
I'll have to admit, it's one of the crappiest parts in the whole car...
... I expected something more elegant from MB.