Coatl exists in the FSdreamteam folder, but It doesn't show up in the addons tab like the addons manager does.
Buildings on other demos will appear fine, but the greystone building scenery won't.
These two conditions simply can't happen together: If you don't have the "Couatl powered programs" menu under "Addons", you wouldn't see ANY buildings in any of the FSDT sceneries, EXCEPT Greystone!!
That's the only scenery that doesn't depends on both Addon Manager and Couatl to run but, instead, will work even without Couatl running, but just the Addon Manager.
Are you sure you don't see the Couatl menu, but you see all the other FSDT sceneries ? If yes, the only possible explanation, is that you are using very old versions, before we had Couatl, that's why they work.
And of course, Greystone works only if you have an installed and activated KORD