The External editor option is not selectable in the Demo version.
But you can still make it work. For instance, you work a bit, save your data and then open up the FS Repaint and load it, then see what needs fixing. Might take a bit longer than just using the external editor function, but it beats having to load up FS every single time and see the tiny little mistakes you make, then having to shut 'er down and adjust, then restart, shutdown, restart, shutdown . . . restart.
