The message suggested you the correct solution:
I received this message upon starting FSX
"The Couatl Scripting engine has encounted an error while loading. Installing the latest Stand-Alone add on manager should fix problems"
There are links to download the Stand-Alone Addon Manager on every "Info" page of every product, and this is also indicated in the FAQ page.
You need to install the Stand-Alone Addon Manager with all the sceneries installed, because it needs to update them so, only the installer ones will be updated.