How do I install the center fin?
When your board arrives from the factory, there is already a screw plate and screw in your center fin box. Unscrew the screw from the fin plate and slide the fin plate all the way to the front of the fin box (towards the front of the board). Set the screw aside for a minute.
On the center fin, there is a metal peg on the back of the fin base. When putting the center fin into the fin box, drop that peg thru the wider opening at the center of the fin box and down into the track. The peg will slide down the track. It's pretty tight sometimes, so we use a rubber mallet to tap, tap, tap it back into position so the fin is roughly centered within the larger fin box.
Rotate the front of the fin base down and line up the hole at the top of the fin base with the hole in the screw plate. Drop the screw that you set aside into the base of the fin and thread it into the screw plate. Sometimes if a really tight fitting fin and you'll just barely be able to get the screw threaded into the plate. Don't worry about it. As long as you can get it threaded, the leverage of the fin plate can suck the fin base down flush.
Note if you are having trouble getting your center fin to fit in the fin box you can lightly sand the edges of your fin's base to get it to fit in.