The most likely cause it's the antivirus, which has mistakenly blocked *some* files, allowing other to be installed, and causing a mess in the installation with old/files mixed up.
- Download and install with the antivirus off
- BEFORE launching FSX, configure the antivirus to exclude the following files from scanning:
YourFSXFolder\bglmanx.dll
YourFSXFolder\fsdreamteam\Couatl\Couatl.exe
- Now you can turn your Antivirus on, and launch FSX
- Note if FSX asks permission to run the Addon Manager and the Couatl programs, reply YES to both
- When FSX loads, check if you have BOTH the "Addon Manager" and the "Couatl powered products" under the "Addons" menu.