the pink "stuff" on the bottom of the heatsink is the thermal goo....BUT, you have to be careful. there is often a plastic lining on it, which you MUST remove before you can mount the heatsink
(sometimes there isn't one though, which is why it can be confusing)
but assuming you take off the plastic lining, or that there is no lining on yours, you can fee free to mount the heatsink and make sure the "thermal goo" is making good contact.
try to seat it only once though, repeatedly removing the heatsink and re-seating it will lessen the effectiveness of the heastink pad.