In 1.7.1, if you press the ??? button, you will have to do a bossfight. Then, when you have beat the bossfight, you will need a pass code for a door. You will find the pass code in the door on the left door, then, you will have to go into the computer and you will find the pass code. When you have gotten through the pass code door, some one will appear and talk, when it's done talking, the game will crash, the you re-open it, then select your save file you used when you was doing that boss fight, the you unlock debug mode!