http://www.ifixit.com/Guide/Installi...1#.T7pCHY5zrlI
See this for instructions on removing the heatsink.
I use Arctic Silver 5 heat sink paste and so does ifixit.
One of the other forum members has suggested Tuniq heat sink paste in one of the posts. I disagree with him, but accept his point that the matching of the heatsink surface and cpu surface is not the best. Tuniq seems like a quality compound, but I have used Arctic Silver 5 on both my Mac Minis, two Mac Book Airs and various laptop PC with no problems.
Just make sure you don't use too much paste (or two little). Do a few web searches on this or watch you tube if you have never done it. Or you can always do one application and then check the spread of the paste, clean it again and then reapply for the final go. I use pure Methylated Spirit (Alcohol) to clean the old past of and clean the surface. Use tissues and cotton ear buds. make sure both surfaces are immaculate.
I follow the instructions available on the Arctic Silver website. Three schools of though in application of the compound: spread it on the surface, or put a bead in the middle, or put a line. I use a line for a rectangle surface and a bead for a square surface. I totally disagree with the spread (Tuniq) method because you run the potential for an air pocket.
Note also it can take a week (200 hours) for Arctic Silver to "bed in". It effectively becomes a little firmer.
In summary, there is a lot of material on the internet on heatsink installation and Arctic Silver (or Tuniq) has application details on their website. Note finally, you can get the paste easy on ebay, and squirt out the first 5% or so of the syringe and throw it because it can be a touch oily with the first little bit (i.e. make sure you get the pure mix because you don't need a lot).