In that case, go to paypal.com on the computer and (even if you use your bank account) update/add current credit card info. Then try to login to app world. (with the login option in the my world section, not through the purchased app) beyond this, make sure app world is the newest version from mobile.blackberry.com and if you're still having problems, then send 200 emails to blackberry mobile support.
Posted from my CrackBerry at wapforums.crackberry.com