i have never used the migration tool to install P3D only addons, just when installing FSX addons into P3D.
Then your FSX registry key is missing. If you reply No to the question about installing into P3D, the installer will try the FSX:Steam folder and, if it's missing, it will try the FSX registry key (which can be used by FSX:SE too, so it will look for it in that folder too). But we can be sure of this, by logging the installer, this way:
- Open a command prompt ( start -> CMD -> enter )
- go to the folder where you have the installer, for example:
CD c:\Users\Yourname\Desktop
Assuming you put the installer on the Desktop
- launch the installer with this command:
gsx_fsx_setup.exe /log=install.log
This should hopefully create a file named install.log on your Desktop. If you have it, then send it to me ( via Email, please ).