The "safe" this refers to is not the safe on the wall, but rather the locked cabinet in the desk in the center of the main room (on the opposite side from the computer console). To unlock it, you have to type a code into the 0/1 keypad above the drawer. The code can be found by placing a red lightbulb you find in the lamp in the corner near the couch, then re-aiming the lamp toward the center of the room to reveal the code.
Once unlocked, open the cabinet and pull all of the gold bars and coins out to unlock the achievement.