Hi Frior
I know what you are going thru, even as a system builder, I get a bit nervous at the processor installation point.
I'll be happy to help if I can, but I'd need some more info first, specifically photos of the heatsink and clamp assy and of the socket on your mainboard.
One point I would make though is that installing a CPU with the mainboard in the case is risky as you may have to apply a fair amount of force to the clamp to get it to lock in place. As most mainboards are held up on pillars, pressing down hard on the heatsink clamp is likely to bend the mainboard, and overdoing that can lead to a whole world of hurt. Personally, I'd remove the mainboard from the case, and fit the CPU and heatsink with the board sat on something which which will hold it secure. I use a sheet of polystyrene about 20" square and 5mm thick which I glued to my bench for this very purpose. In fact, I tend to add as much to the board as possible before installing it (memory etc) to minimise the amount of messing about I have to do inside the case.
Apologies if this post oversteps the mark 'cos I'm a newbie here...
Jabo