Submitted by: David K.
On the main menu, use the keyboard arrow keys, the left gamepad thumbstick, or
the D-pad to enter the Konami code: Up Up Down Down Left Right Left Right B A
You will receive the "God Mode" Steam achievement, all the levels in the "New
game" menu will unlock, the console will be enabled (press ~ to toggle it), and
you will be able to open the singleplayer campaign levels in the level editor.
Note that it's recommended to duplicate the campaign levels before editing them.