From your description of the problem, it's not that Couatl is not loading, it seems the INSTALLER doesn't even start. And, in any case, there's no such thing as a "Couatl installer", maybe you confused it with the "Couatl_updater.exe", but you are not supposed to run it manually.
The Couatl is included with every FSDT installer, the easiest way to "reinstall" it, is to download and install the Stand-Alone Addon Manager.