Adding a Credit Card
While signed into Xbox LIVE with your gamertag, move to the "My Xbox" tab and go to "Profiles." Use the "Manage Account" setting and choose "Change Payment Options." Select "Add a Credit Card" and enter all the required information exactly as it appears on your credit card.
Adding Points to Your Account
Microsoft Points can be purchased from the download screen for any DLC in the Xbox LIVE Marketplace. After selecting "Download," press the "X" button to add points to your account. Microsoft Points can be purchased in 400, 800, 1,600, 4,000 and 16,000 point increments.
Using Xbox.com
With a valid Windows Live ID, you can also sign into Xbox.com to update your Xbox LIVE account information, including adding and changing credit card options, and purchasing Microsoft Points. Additionally, you can search for and purchase DLC through Xbox.com, and the content will download to your console the next time you power it on.