If the Addon Manager doesn't show up in the menu, it doesn't have anything to do with registry, scenery.cfg, registration (or lack of), activation, etc.
It's usually more a problem of a corrupted FSX installation, which has a non-working Simconnect because of this. Since the Addon Manager is a Simconnect client, and it creates its menu using Simconnect, if Simconnect is not working properly, or it's not installed, or it's corrupted, the menu will not be created, and the program will not run. If the program doesn't run, no buildings will show up and nothing will work.
Simconnect comes with FSX, and can only be installed with FSX. There are also 3 versions of it: RTM (the original that came with FSX), the one that comes with FSX SP1 and the one that comes with FSX+SP2 or FSX+Accel.
Our products will only work with the latest one so, if you have FSX only or FSX+SP1, it will not work. You need to have FSX+SP2 or FSX+Accel. Also, if you uninstalled SP2 or Accel, the right version of Simconnect we need might have gone. In this case, you need to reinstall either SP2 or Accel.
Another reason why the Addon Manager might now work, is because of a conflict with another module. We had issues in the past with FSCopilot, which stopped the Addon Manager from working, if it's loaded before it, but this should be solved by our installer, because it puts the Addon Manager before any other module in the DLL.XML file.
Try these suggestions first.