When you unlock your doors using the central locking system, the fuel door lock (which is located inside the trunk next to the fuel door) releases, letting you get in to fuel the car. When you lock the car using the central locking system, it pushes out a little pin that locks the fuel door shut.
If you need to get in to the fuel door and the release isn't working, you can reach through the little hole in the trunk liner and manually pull back the solenoid arm to release the fuel door.
If your fuel door doesn't lock at all, the central locking isn't working, the fuel door release isn't working, or the release is broken. Maybe that little pin you found?