Be sure you install the Couatl 3 Public Beta as the LAST thing you do, AFTER GSX.
Enable Logging in the "Troubleshooting" section of the GSX Settings, than restart FSX, go to the same airport you tried before, let the error appear, and see if GSX created a Couatl.LOG file and eventually a Couatl.ERR file in your %APPDATA%\Virtuali folder. Send me both files, or at least the Couatl.LOG file, if no .ERR file has been created.