I have been using fsx for quite some time but this is some unique problem I am facing. I currently have my computer in two partitions windows 7 and windows 8, earlier I only had windows 7 and use to play on that but recently had to format my system. After formatting when I again tried to install it in windows 7 it was giving error message that "unable to load gameux installer.dll" i tried ti fix that dll, and it was reinstalled but I was still facing the same problem. So i installed it in windows 8, but here again I did not have enough space in my drive where windows 8 was installed so i had to install it in the drive where windows 7 was installed and it worked perfectly fine,but the problem here is that none of my add ons are showing up in fsx neither the planes nor the scenarios. Earlier they were working just fine but now I don't know what happened. Can someone please help me!!!!
This is what Microsoft has to say about your problem.
Personally because FSX scatters folders in different areas of the hard drive one (or more) may be in one OS and some may be in another.
Let us know if it works for you.