When you arrive, just press the intercom button on the left hand side of the screen. Make sure you have a copy of your booking confirmation to hand when you arrive. A member of staff will then process your booking and let you in, so you're free to park up and take the short walk to the terminal.
Just drive to the exit barrier and press the intercom button in the top left corner. You'll be asked to give your booking reference again (so you'll need your booking confirmation) and then the barrier will lift to let you out. Again, don't put your credit or debit card into the barrier. If you are late returning, you will need to head to the information desk before you return to your car.