I don't think paypal will actually show the available money on the card itself.. but I'm not sure (just a guess) call the number on the back of the card to double check that you have money on it.
It also could take a day or so for paypal to verify that the card has a positive $ amount on it.
I've never used a pre-paid card with paypal but I do use a credit card and it doesn't show the balance of the card itself but rather the balance of the amount I charge to it.. So figure out what that balance is for

Hope that helps, but if not just contact paypal support they're pretty decent at helping ^^