Do you have like proof of buying it in the form of a confirmation letter or something? You will need to take that to the pre-booked ticket kiosk where you will get a big entry ticket, where you will go through the turnstiles. Then turn immediately left and the annual pass office is there, where you will get it. Most of the time there is a big queue though. This is how I do it with Tesco clubcard voucher so I think it will be the same.