Once you have earned the Mega Bit by buying it from the shop after it becomes available, you will have access to a secret room in the tutorial world (Red Dozers' Training Course). This room contains a 4-digit combination lock.
Before you can properly use this room to enter in secret passwords, you must first enter an unlock code. There are 3 levels of Unlock Codes, each unlocking more passwords than the previous.

To enter an Unlock Code, you must first reset the combination lock by selecting one of the digits and hitting start while the drill is inserted - this will reset the code back to 9999. Once this is done, you must set each of the digits of the Unlock Code to their correct number in the correct order. If you change one of the numbers out of order, or if you connect to and alter the same number twice before completing the unlock code, then the code will not work and you will have to reset the combination again by pressing Start.

Once the fourth number is correctly set, the combination lock will auto-reset to show that you have entered the code correctly. You may then either enter another Unlock Code, or enter an Effect Code of equal or lower level.

Unlock Codes stay in effect until you leave the room. If you re-enter the room at a later time, you must input the Unlock Code again to use more Effect Codes.