Find the 'X-Plane' row in the Export Directories table, click on the 'Browse' button to find and
select your X-Plane 'root' folder. Press 'Ok' when done.
Go to the 'Settings' tab and press the 'Save Config Options' button.
Shut down, and restart Fsbuild for the new settings to take effect.
Go to the 'Build Options' menu, and select the 'X-Plane FMS' menu item.
Build the route, the flight plan will be exported to your X-Plane/Output/FMS Plans folder.