So he installed it by moving the FSX folder to the desktop and by clicking "Browse Sim" and selecting the folder.
NOOOOOOOO!!!! ( imagine this to be uttered like Luke Skywalker, when he discovers Dart Vader is his father... ).
This has totally ruined the FSX installation. You just can't "move" applications like this, when a program is installed, its location is written in the registry, but if you move it, the registry won't match the installation, and nothing will basically work anymore, you'll have countless of issues for all addons, which all must rely on the registry to find the sim.
First, you should immediately put back FSX where it was. And, as I've said in my previous message, be sure to check the registry, as explained in this post, and verify is correct for your FSX:SE installation:
http://www.fsdreamteam.com/forum/index.php/topic,14166.msg104727.html#msg104727But I now think that, if you don't have many other addons installed, it might just be easier to remove FSX:SE altogether, then reinstall it from Steam, and launch it at least once, so it can set up itself correctly, before reinstalling GSX: