As the message says, installing the Stand-Alone Addon Manager for FSX WILL fix this.
You must do it with ALL the sceneries installed, since the Stand-Alone Addon Manager installer will have update some files for the sceneries, but will only do for sceneries that are actually installed.
Note that, if your installed sceneries are VERY old, meaning you installed before we added the Pyhton features to them, installing the Stand-Alone Addon Manager will not be enough, you'll have to reinstall the scenery using its full installer, something you'll want to do, since it means your sceneries are really outdated.