i have the late 2006 macbook model. I just picked up my macbook last week from genius bar. They replaced the topcase because the plastic border of the macbook was chipping off, and they said the arrow popping up was a defect as well. It popped off, but same as you I tried to put it back on like the YouTube videos told me to and I broke the hinge in two. LOL.
What I learned is: it's worth a shot to take it to the genius bar and see what they can do for free, and get an estimate of how much it would be to replace the arrow key.
Good Luck