Well it wouldn't be unlockable if I couldn't do anything. And I haven't put anything back together yet :p
First you need to clean the old thermal paste off. You may use alchool and paper, but be careful. The processor should look like this (the picture if from my old graphic card):
Then, you must buy thermal paste, like Artic Silver 5, apply just a little bit, and then put the cooler on.
Here you have a link to a tutorial:
http://www.hardwaresecrets.com/article/274