You put the metal U bracket back in its place, and extend it all the way down until it hits the back of the key...it should be hinged on the backside of the key (like the bottom facing side). When you swing it down the 2 hook sides should be facing as far down as they can...drag the key backwards over where it should be, all the while having the 2 hook parts on the U arm grab their corresponding places on the keyboard. Once they are in their place you just push the key down until it snaps in place.
Just had to replace my keyboard on my dell 2 days ago...spilled milk all over the damn thing
EDIT: Wow that REALLY doenst help at all...made sense to me when I typed it since I just had to do this myself. Sorry, not much help here