The car park has automatic number plate recognition, so just drive up to the barrier and wait for the machine to read your number plate. It will then print a ticket with your registration number on. Then just drive in and park. If after 10 seconds the machine still hasn't given you a ticket, press the help button to speak to a member of staff. Don't press the ticket button at any point. If you can't or prefer not to walk to the airport, just go to any of the bus stops and press the intercom button to call the bus.
If you need the bus to come and pick you up, just call the car park using one of the intercoms at the terminal. There's one on the pay machine (where customers who've not booked have to pay) and one by the bus stop outside the terminal. When you get back to the car park, just drive to the barrier and put your ticket in the slot. If the barrier doesn't lift, press the help button to speak to a member of staff.