OK, I think I know the problem, done lots of these by doing pre-delivery inspections. The plate bracket (which is what it's called BTW) just goes below the bumper, has the 2 screw holes in the botton corners for the lower plate holes. Then what I do, although I've seen nothing official, is to mark and drill the holes for the top holes in the plate, right into the bumper itself, then put the top screws in. They hold really well, I always use the MB screws that the dealer should be able to supply you with. Sometimes the bracket comes with the screws.
I can see why you think it's for a European plate, it's actually the right width, but the plate extends up over the bracket when installed. It looks really bad without those top screws, the plate starts pulling out away from the bumper.