The FMU comes with a 3D printed enclosure, which still has to be assembled. It consists of a top part, a bottom part and a tiny reset button.
First, make sure the reset button is installed inside the enclosure. It should fit in a hole in the side, next to the hole for the micro USB connector.
Now, the FMU can be installed into the bottom part of the enclosure with four short screws (7 mm). It could be the case that you only have one type of screw with your case, which is fine as well. Just use four of the included screws and be gentle when tightening them.
After the FMU is installed into the bottom part, you can put the top part on and turn the whole enclosure upside down. You need two short screws (7 mm) and two longer screws (about 10 mm) to keep the two halfs together. The short screws should go in the holes close to the servorail of the FMU. The longer ones go in the holes near the SD card slot. Again, if you have only one type of screw with your case, that is fine as well. Just be gentle when tightening the screws.