On the visa website (where your account info is shown) you have to click on "Manage Disney Rewards". It will take you to a rewards summary page. Then click "Redeem Rewards". On the next page, click "Order Rewards Items" and order a Disney Rewards Card. From there you can choose how many Rewards Dollars to transfer to that card. When you receive the card in the mail (watch for it, because it comes in an unmarked envelope) you can call Disney and tell them you want to make a payment on your vacation using Disney Dream Reward Dollars and give them the number on your card. It should look like a gift card.
Good luck and hope that helps!