You have to first unregister any credit or debit card you have on file to make payments from and then register your paypal account. To do this, go to Manage My Account at the top of the screen, it should bring you to your account page on the 'Account Overview' tab. Click on the third tab, 'Settings and Preferences' and you should be able to change payment type from there.
It's a pain in the **bleep**, but this is the only way you can pay via paypal. Also, when you select a top-up amount, you will be charged mysterious fees/taxes (I'm currently having problems since I specified the sum of my paypal balance), so leave room when you make your payment.
