By clicking on the 'credit your account' link you will then be taken to a page where you can select the number of hours that you wish to purchase. Currently you can select from 1, 2, 3, 5, and 10 hour packages.
Once you have selected the hours that you require, you will be taken to a secure Paypal payment window, where you can choose to make payment with your Paypal account or a credit card.