I recently received one of these letters. You have to call the 800 # for guest relations and give them the number that is on your letter. From what I understand, the letter at the end of the number specifies what they are able to offer you.
They read you a list of choices, you pick 2 but only get 1 (incase they are out of your first choice) - the choices I was given was: wine (red or white), champagne, chocolate ship, fruit basket, 4 bar credits, 1 photo credit, bon voyage decorations. (She didn't know the type/brand of wine or champagne). She said it would be in our cabin upon our arrival.
If you do a search and type in 45 day letter you will see another of the same topic that includes the customer relations addy you can write to.
I did, and got my letter, and in just 4 more days I will be calling to get my gift.