You have to have a Premier or Business account with PayPal before the option for credit card payment will show up.
If you already have a Premier or Business account, you probably have the relevant setting turned off. Go to Profile -> Selling Preferences -> Website Payment Preferences and look for "PayPal Account Optional." Make sure that box is checked.
If you can't find the box, click through lots of links for a while. PayPal enjoys rearranging their website a lot.
P.S.: If you are using Sandbox mode, it won't show up. You have to be using the real Express Checkout.