Ok - so I've done it - bought my app from Blackberry World.
But I couldn't - and I tried and tried!
This is how you do it - go to BlackberryAppWorldStore - there seems to be a problem on the playbook - so go on WITH A COMPUTER. Sign in. Find your device name (top right) hover over it and "account settings" and "payment preferences" pop into view.
Click on "payment preferences" and log into your paypal account or set your credit card setting there.
Go back to your Playbook and purchase your app.
Works like a charm!
Who would ever KNOW for God's sake!!!!!!!