How can I add more money to my iTunes Store account balance?

When open iTunes and click on the iTunes Store, I can sign into my account by clicking on the Sign In option on the far right. But, I can't find any button or option where I can use a credit card to add money to my current balance on the iTunes Store. I must be missing it somewhere. I have a credit card on file with my account, but I can't find where I can use it to add more money to my account. Any suggestions? Thanks.|||you can only add money to the balance by buying an itunes gift card at any store and redeeming into the account. otherwise, when you try to make a purchase, it will be deducted from your credit card. screenshot: http://tinyurl.com/faqhelp