This happens when the game tries to run at a higher resolution than your computer can display. To fix, press Alt+↵ Enter to force the game into windowed mode (you may have to click its icon on the task bar and do this while the screen is blinking), then you can go into the settings and change the resolution, at which point you can go back to fullscreen. Note that you may have to do this every time you start the game.
A level editor made by Andrew Start is available here.
Detailed instructions by Tim Remmers are available here.