well here is a brief summary... Mr. lazy:
Credit cards on PayPal accounts are used as a backup source of funds, and you can use your credit card for purchases directly through PayPal. When you get the the Paypal screen that shows the source of your funds, you have to click on the little blue link that says "Change". You'll then be taken to a screen that shows a list of possible payment options. Your credit card will show on that page. Check the option next to it and click to the next page. The next screen will try to talk you out of paying with your credit card and ask if you're sure you want to do that. Click 'Yes'.
Now your payment through Paypal will be charged to your credit card.
I don't know if you can add funds to your account using the credit card, but you can use it to pay for something. They will charge your card for some fees.