It's much easier if you just do the following:
- Uninstall everything, both FSDT and Flightbeam. Reply YES to the question "Do you want to remove the Addon Manager ?"
- Reinstall all the FSDT sceneries, using ONLY the current version of the installers. DO NOT install with older installers.
- Reinstall all the Flightbeam sceneries, using ONLY the current version of the installers. DO NOT install with older installers.
Now the installer give you a choice of installing outside the sim, it's best if you do it that way. The suggested location is FINE, unless you don't have enough space on C: Installing on another drive is perfectly fine too, installing INTO the sim, it's probably the worse choice.