Click on the My Account tab
Click on the Profile subtab
Click on the Credit Cards link in the 'Financial Information' column
Enter your credit card or debit card information
Follow any additional on-screen directions
As far as I know you wont be able to add funds to your paypal accounts; however, if you are going to pay someone you can then choose to pay via credit card and that will be instant.
The way credit card treats withdrawals and purchases is different. If you add money from the credit card, it's like a withdrawal of cash and the charges will be quite high on top of there being an interest.
Just send money to the intended payee. If there's not enough funds, Paypal will auto charge to the card you have on file.