Move the mouse to the crystal above the "Apocalypse" logo at the player or level selection screen. Then, assign a numerical value to your mouse buttons, as follows.
MOUSE BUTTON - NUMBER
Left Mouse Button - 1
Right Mouse Button - 2
Middle Mouse Button - 4
Using those values, you can create a "cheat number" by holding down different mouse button combinations. For instance, to create "cheat number" 3, hold the Left Mouse Button + Right Mouse Button (1 + 2 = 3). The cheat mode levels have "cheat number" values from 1 to 7, although 5 to 7 are the same. The higher the cheat level, the better the cheats becomes. On all cheat levels you can choose any planet to start at. At cheat levels 3 and higher, the shield charge increases. At cheat levels 3 and higher you will also have rapid fire. One way to get cheat level 5 (the best) is to press Z to continue instead of SPACE when you insert the levels disc.