When you arrive at the car park, to enter, wait in front of the entry gate: the license plate reader will register your car and the ticket will be issued automatically. This ticket is already prepaid and has your Parclick reservation information (if you press the button, your reservation will not be valid and you'll be charged the rate price, when your reservation is already paid through Parclick)
If, when you arrive, the number plate recognition system doesn't recognise your car and fails to give you a ticket automatically, just press the button, take a normal ticket and go to the office with your Parclick reservation once you are inside.
Go to the control booth (or ATM) to validate the ticket that you got at the entrance. Location of the customer service booth: follow signs to Aparcamiento P-2, which is at the end of Terminal 2, on the first floor, before taking the lift up the parking area. Only in case you have exceeded the time of your booking you will have to cover the difference in price in the car park. Then, drive towards the exit with your vehicle and use this ticket to open the barrier.
Avenida de la Hispanidad, Aeropuerto de Madrid-Barajas, Terminal 2 Llegadas