They used to put a card in your stateroom, but on our last cruise there wasn't one. I went to Guest services to check--the credit was there, but they said they had discontinued the cards.
I'd still check the first day or so...if enough people bug Guest Services, they'll re-institute the cards!
I was told by TA that it doesn't show up on the confirmations even though they know it is there (hmmmm...I may want to re-check on that one.)