Hi, I m totally new to this forum, but here goes. You need to go to your FSX Installation folder eg: C:\Program Files (x86)\Microsoft Games\Microsoft Flight Simulator X Folder. Right Click on SimObjects Folder and select properties. Click on the Securities Tab. There you will find a list of users. Click on the edit button to change the permissions. Select the username you are logged in as and allow yourself full control. Click Apply and Repeat the process for the user called Trusted Installer. Press ok when done and your problems will be solved :)
PS I read about something similar in the PMDG Manuals but changing it on the folder is better because you never have problems editing a aircraft config again ;) And it works!