1. Go to the Space Brain Transplant.
2. Move your hand to the upper right corner to the keypad.
3. Enter "296145" (if you enter a wrong digit just press random numbers so that it resets and you can start again). After hitting "2" the next digit should be slightly inward making it easy to find the next one.
4. Optional: Immediately after entering the digits, grab and hold the radio to get another achievement: "We're Going On An Adventure!"
5. You've unlocked "Enter The Code."