Ok, figured it out. The antenna plate is mesh-like with a bunch of holes. I slid the antenna plate into position (as close as I could get it) and lifted the front end of the plate (at the apex of the curve) with a small screw driver. I was then able to push the plate forward a little more and the plate poped into correct positioning.
Even simpler, quicker and safer, no tools required:
Turn the antenna grate/plate/cowling upside down but where it's supposed to go on the lip of the Mac mini case. Push down gently with your thumbs around the plate near the rim. Turn it the right side up again and it should now snap into position so the screw holes line up. I'm posting this in gratitude to all the others who posted solutions when I was in despair trying to figure out how to get it back together.