That extra post is for the neutral safety switch. There is a single connector near the solenoid that would have gone to that post. It keeps you from being able to start the car in gear. It sounds like either you have a bad ignition key cylinder or a bad ignition switch. There is a small box that is under the steering column that is connected to the ignition cylinder via a rod. You can try jumping the solenoid for now. Turn the key in the run position and then using a pair of needle nose pliers, touch one side of the pliers to the large terminal that goes to the positive terminal of the battery and the other side of the pliers to the S post which should be the small post right next to the positive post. You can buy new key cylinders at the auto parts store. There aren't very expensive, but you have to remove the steering wheel to replace it.