1. Select 'Account Settings' from the Main Menu on the left
2. Switch to the 'Billing' tab
3. Select the 'Update Credit Card Info' button on the right of the page
4. On the window that opens up, select 'Check Preapproval Status'.
You will now be taken to Paypal with two options...
1. Enter in the login details for your existing Paypal account
2. Enter in your Credit Card details, and a Paypal account will be auto created for you.
If you haven't created a Paypal account before, you would want to go with option 2.
Please follow the onscreen instructions. Once your billing information is in, your Billing page will be auto updated and you may switch to any of the other Subscription Plans available.