The easiest way to install ANY 775 Heatsinks is out of the case. Obviously rid yourself of ESD then with one hand support the bottom of the mobo right around were the pin will come through, then push it in. Repeat in the diagnal pattern and viola! This is the best way because all the pressure is being countered by your other fingers so 1)your mobo doesnt flex 2) all the pressure is being focused on the pin causing it to actually go in easier as well.
Also once you remove the HSF after the paste has setteled it's usually a good idea to remove and re-apply, I have done this without re-applying but obviously the best possible transfer wont be attained. Plus the SHSF is absolute garbage. I picked a CM TX-2 for $8 at the Egg.