The usual questions:
- Do you see the "Addon Manager" menu under "Addons" ?
- Do you see the "Couatl powered products" menu under "Addons" ?
If not, the most likely cause it's almost always the antivirus so, you should do this, exactly this, don't skip any step:
- Uninstall the scenery. Reply YES when asked if you want to remove the Addon Manager and the Couatl Engine. Reply YES to both questions.
- REBOOT Windows! (please DO IT)
- TURN OFF your antivirus. Don't just exit from its interface, but turn off the real time protection. Actual wording changes between products, however, it's the option to scan every file that gets open in real time, without user intervention.
- Reinstall the scenery with the Antivirus TURNED OFF. Don't launch FSX yet.
- Configure the antivirus to exclude the following files from scanning:
YourFSXFolder\bglmanx.dll
YourFSXFolder\fsdreamteam\couatl.exe
YourFSXFolder it's usually C:\Program Files\Microsoft Games\Microsoft Flight Simulator X, if you installed it in the default folder.
- Now you can turn the antivirus back on, and launch FSX.
- Check you are seeing BOTH the "Addon Manager" menu AND the "Couatl powered products" menu under the "Addons" menu.
