If you have problems with missing FSX/P3D registry key then download and use this little utility to fix it
https://www.nmgsimul...registry-fixer/
Extract the file to a folder and run.
Warning
We didn't make or verifiy this software.The entire risk of using this software is with you. This means that if you “stuff up” your flight simulator installation, it will be your problem! We do not take any responsibility.