I upgraded to a athlon 64 bit a few weeks ago but didn't have a problem getting the heatsink to fit. I have a Abit board and the heatsink fits with the black leaver towards the bottom of the board. I put the non lever side bracket on 1st then clipped over the lever side bracket. Then just flip the lever so its parallel to the heatsink and press it down until the catch clicks in place. Maybe you've got a faulty lever mechanism.
Don't know if this will help:http://search.amd.com/query.ht...stal...tion+guide&rq=1
click on the 4th link down and it should open a pdf with installation instrictions.
Let me know how you get on with the Zalman heatsink as I'm thinking of getting one. My core temp is quite high at around 43C idling.
AMD 3700+ San Diego, Abit AV8 Stk 939, 1gig DDR400