Sorry to say, but I also have lost all my buildings (PHNL, Hawii 1+2, KDFW, LSZG) after GSX (trial) install.
First, download install the Stand-Alone Addon Manager after all the scenery NOW, and see if it changes.
If not, open the Notepad and type the following on a new blank file:
logFile=couatl.log
(please note logFile is written beginning with a lower case l and a capital F )
Save the file as couatl.ini in the FSX\fsdreamteam\couatl folder, so it stays at the same level of the couatl.exe
Launch FSX, go to any FSDT airport, when it finished loading, exit from FSX, check if you have a Couatl.log file in the fsdreamteam\Couatl folder, and post its content here.
If you don't have the file, either be sure you followed this to the letter, or try running FSX As Administrator.