My experience was a bit different than Agnes'. I used points to buy tickets last April; I had to find out the final cost of the tickets at the box office (used my MRB coupon), go to the TR desk to get a credit slip for said amount, then go back to the box office. A bit of a pain, but not too bad. I'd love to be able to pay straight at the box office...
But on a related note: can you charge tickets at the box office to your room? If so, that's a simpler way to do it.