Take off the third brake light (just the two phillips screws, then it slides straight back toward you).
There are 4 allen head screws holding on the trim piece below the spoiler - take those out and the trim piece, then you can get to the four screws (nuts and bolts really) that hold the spoiler on - two on each side. You should see the philips heads from below; once you get the trim off, you can get to the nuts on the inside to hold them with a wrench while you tighten.
You'll see one pair of screws once the trim piece comes off, they'll be inside the end of the side trim. The other is a little further up, and you need to gently pull the side trim back to get to the nut.
If they're missing entirely, look for short (15mm) machine screws, I think they're about 4mm diameter, flat head (like wood screws designed to go in flush with the surface) You'll want to use stainless if you can find it. Use a lock washer under the nut to keep things secure.