NOTE: If it doesn't immediately work; try pushing it in other directions than down/towards yourself... No tools or disassembly needed. Press down somewhat hard and push it towards yourself. This worked for me after it was "broken" when I got it back from repair. Comment from T-rex: "If it's not working for some of you, there's a Torx T6 screw located either underneath the battery of your MacBook Pro. If you have a MacBook Air, then it will be located underneath the aluminium cover of the laptop. You will see it, no need to remove the battery. If you tighten that screw too much, the trackpad won't click. If it's too loose, it might rattle more than before. If you're going to adjust it, make sure you keep checking the trackpad by opening the MacBook and clicking the trackpad."
