There are two ways to add a card to your account:
Go to your Account Settings under My Ocado and select the 'Stored payment details' box. Click on the 'Add new card' button and enter your card details.
You can also add a new card at Step 5 of the checkout. Simply click on the 'Add a credit or debit card' link and enter your card details in the box provided. Then, when you move on to Step 6, you can store the details for future use.