The right arrow key came off my friends 13" unibody macbook pro too - maybe it's a common thing. I'm afraid it's needlessly complicated. Anyway you have to slide the x-shaped hinge into the central holder, then press down on the sides so it clicks in. Then slide the side parts of the hinge into place on the key, and push down on the centre. It should click into place. If it isn't working, make sure the hinge is the right way up.
okay, i've tried all of this, can someone give me a video of putting a arrow key back onto a keypad, or very easy instructions of how to do it. borisiii, you gave me good instructions, but they didn't work. oh and btw, my laptop is the late 2007 version.