MetaFic

They should fit in the old screwholes all things normal, it's always possible your motherboard manufacture took some non-standard liberties with screw hole size if the pins didn't exist when it was made. I think it's unlikely though assumeing the pins match up with the holes. I'd make sure your pushing them all the way in, it took me a lot more force then I assumed when I had to deal with them on my home pc. It was worse then the old 478 sockets but I also had a bigger heatsink for them to affix.
Assumeing I remember this correctly you turn them inwards towards the heatsink which unlocks the pin, and then pull up to make sure it's not pushed through. When you have it all centered like you want it push the pin as hard as you feel safe doing and then turn it to the locked posistion, which should be outward, with the arrow pointing at a 90 degree angle from the heatsink. The 3rd/4th ones should be harder to push in if they are actually pushing through the hole all the way, though that may not be as bad with that heatsink as it was with mine. After your content they are in look at the other side of the motherboard and check to make sure the pins actually pushed through.
If you already did all this right then your guess is as good as mine, but it took me a good 45 minutes to figure the pins out with having to repaste it and all and being to stubbron to have to google how to use some silly push pins. Even once I cleaned everything off and fidgeted with it until I figured out how they worked I was still confused for a bit because of how hard I had to push. Good luck, hopefully my wall of text helps some.

I did it with the motherboard in the case because in theory this was the least amount of hassle. Once I figured the thing out it ended up being a really good idea because the metal case braceing the motherboard helped with the having to push so hard. Again though my heatsink was on the large side, that looks about the stock cooler design of the i7920 which doesn't take any abnormal force to get pushed in. The only problem would be if the case doesn't allow easy access to the back of the motherboard to see if they pushed through or not. The checking is really the important part, I think the way the push pins where used on my heatsink is in a way they where not intended to be so I wouldn't expect to have to use the same force I did.