Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
If nothing is broken, it should just snap into place. Some gentle pressure should do the trick. On thing you can attempt is to attach the 'bottom' of the key first, such that you're holding the key at a slight angle with the key tilted toward you as you place it into the slot (assuming you're in a normal position facing the screen sitting in front of the laptop).
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.