You are not supposed to change the security permissions for that file since our installer obviously set the correct permissions for its folder when it starts.
But the access denied might be caused also because you try to change permission (which is not needed to begin with), when the sim was still running, or if was still closing, or if Steam is running.
Try to do it after a Windows restart.
Also, the fact you are using a folder *inside* the FSX folder, seems to indicate you are using an old version of the Addon Manager, since the new one defaults to (and it's BEST to install there) its own Addon Manager folder under C:\Program Files (x86)
- Restart Windows
- Uninstall everything you have from FSDT. Reply YES to the question "Do you want to remove the Addon Manager" ?
- Download and install the CURRENT installers. Don't install anything using outdated installers.
- Accept the suggested C:\Program Files (x86)\Addon Manager location, which is a perfectly sensible choice, if you have enough space on C: