I've installed heatsinks on AMD CPUs enough I can give some help:
After the thermal grease is on the core, set the heatsink on top of it
completely flat. Then hook the non-screwdriver side of the clip into place (make sure it's completely hooked), and grab a small flat-head screwdriver. Use it to push straight-down on the other side of the clip until you hear it "click" into place.
ABSOLUTELY DON'T SET IT AT AN ANGLE!!! That's the shit that usually cracks the core and destroys the CPU.