To avoid graphical glitches, set machine=vgaonly in your DOSBox configuration file (please see the supplied Game Info Card for more information).
Level select: type in the word "BILLANDTED" on the level code screen. Now the message "Incorrect Code" appears, but the cheat mode is actually still activated. Start the game, and you can now press the '5' key on your numeric keypad (right side of the keyboard) to get to the next level.
Description by VGTips