Hi, Jaime! You have to enter your credit card information for a couple of reasons. First, most meals have a credit card guarantee policy. If your party is a no-show, you will be charged a fee for each person, and it will be charged to that credit card. If you attend the meal and use your Disney Dining Plan entitlements, there will be no charges to your card.
Second, there are a few meals at Walt Disney World that require Guests to pre-pay for the entire meal, even if you have a Disney Dining Plan. When you arrive for the meal, you can pay for it using any method you prefer. You can leave it on the card with which you originally booked the experience, or you could choose to pay cash, use a different credit card, pay with traveler's checks, redeem Disney Dollars or Disney Gift Cards, or, as in your case, utilize Disney Dining Plan credits. Simply tell the Cast Member serving you how you'd like to make your payment. Since you have a Dining Plan and want to use it instead of your credit card, the Cast Member will deduct the Disney Dining Plan credits from your entitlements and refund your credit card. Make sure to bring that original credit card with you to the meal. Happy planning and have a Magical vacation!