Op, try to snap it back together with your finger. It might go in is you snap if straight down. Do not use a screwdriver to try and get it in, you will forever scratch the front of your Mavic. If you do watch the above mentioned video, you can stop at 5m32s, he has the top off by then. Be careful when lifting the back of the top off, the GPS/compass cable is attached and you should detach that with a tweezers before lifting the rest of the top cover off.
Also, it looks like he has had his apart a few times. Taking it apart for the first time is sometimes not super-simple, it can be a wrestling job to get the left and right side to let go. Dont worry about breaking it just pull both sides off then the front will release with a forward motion.
When reassembling, GPS plug in first, then the very front body clips, snap in the front then try to snap in the 2 sides behind sensor eyes. Sometimes one side goes in and the other doesnt, and you have to take it off, and retry. And sometimes it takes alot of force to snap the sides in, but it will eventually go in and align.
As he mentioned in the video, dont forget to put in the GPS plug first. If you forget, you will get a compass error message and a GPS error and then you will have to take it all off again and reinstall.