If it has been used on a PayPal before and you're doing the exact same things as before there isn't much we can do about it. You'll need to contact PayPal and ask them to assist you in entering the card number.
If you live in the united states or canada you can also go to the store and purchase a "Greendot Money Pak" which is like a gift card for your self. You tell the cashier how much you want to put on the card (Plus a 5$ fee) and then you can add the money to your paypal. To add it simply go to "Add funds" and then "Money Pak." There you need to add the code on the back of your card and the cash will be credited to your account.