did you try "jiggling" the bracket to make it fall in place before trying to put it on the processor ?... is the processor seated correctly ? You lifted the arm and moved it up and away, set the processor in it the correct way and then lowered and locked the arm ? Does the stock heat sink fit right ? none of the processors pins are bent keeping it from seating right ?
As far as I can see there is no back plate requirement for this cooler. The two clips that lock onto the AMD socket just screw onto the heatsink. The Intel fitting has a plastic frame that is used with the push pins.
you can attach the clips to the bottom of the heat sink loosely if you can get to the screws after it's sitting on the processor.and then set the heat sink in place on the processor. Then get the hold down clips, slip them in place, screw them to the upper tabs... then tighten the uppers-tight....... -then alternate between the two lower retainer screws until the heat sink it held down tightly.