At some point you will be given a task to go to Darwin Village. The first thing you need to do before going for this achievement is make sure Rose is not in your party. Second, you should go to the Ranger Citadel Museum and inspect Finster's head. You will notice it mentions a word etched into the eye socket, "PROTEUS."
Now, when you get to Darwin, you will have to fight through a wave of mutants. If you have a good lockpick or brute force skill you can go to a gate towards the right of where you came in and find the door, otherwise you must fight through another group of mutants.
At the door, select the keypad and you will have the option of inputting the code. Now you should notice the keypad has numbers AND letters, so what was that word on Finster's head? PROTEUS. Use that to figure out the code, or click the spoiler tag if you are too lazy.