It's obviously impossible to help you with such a generic description of the problem so, before anything else: which is the exact error message you see ?
- Are you using the current version of the Addon Manager ? If not sure, download and install the current version of the Stand-Alone Addon Manager, after you installed the scenery, with the scenery installed.
- Have you seen any error messages when installing the Addon Manager ?
- Have you turned OFF your antivirus when installing the scenery and/or the updated Addon Manager ?
- Have you configured your Antivirus to ignore the following files, before trying to run FSX again ?
\Your_FSX_folder\fsdreamteam\Couatl\couatl.exe
\Your_FSX_folder\bglmanx.dll
- Have you seen the question about trusting the Addon Manager when you installed the new version ?
- Have you replied YES to that question ?
- Have you done the same for Couatl ?