I downloaded the App World icon and when I went to buy something, it's telling me there's no credit card associated with my PayPal account. I know for a fact there is since I just bought something off of Ebay recently.
Here's what I would do:
1. Go to PayPal.com from your PC and confirm 100% that your credit card info is there and current.
2. Go back into App World into My World. Click MENU and select Log In. Re-enter your PayPal associated email & password.
Always a good idea to quickly double-check.
Have you upgraded your OS or App World? If so you must Log In again following those types of actions.