The stock heatsinks come with a thermal pad which melts to the processor once heated up and solidifies once cooled down. Using some 99% isopropyl alcohol and something with an edge, but is soft like a credit card, you can scrape off that pad. I suggest using something plastic with a good edge so you don't scrape your heatsink. You start putting scratches in your heatsink and it'll lead to warmer temperatures, so try to keep the base as smooth as possible.
AFTER you've gotten that thermal pad off, THEN you put on some of your Arctic Silver 5 and make sure to go to www.arcticsilver.com
to read their instructions on how to properly apply it