We were there a day early so we went the day before to exchange our voucher for the actual ticket and wristband. If you go the same day, our letter said we could collect our tickets between 3 and 6 pm - you go to the ticket desks (there was no line there when we went) and you give them your letter/voucher and they give you the real thing. Is your name on the purchase or your friend's name? You could write to the people who sent you the letter/voucher and tell them about your situation.
cheers man. luckily there is always my name included on the letter as well, so hopefully it will go smoothly through...