If the last time you have paid an invoice was under "PayPal" as payment method, then you should have it enabled by default.
However, if you decide to start using automatic payments with PayPal later, you can follow these steps:
Log into the client area.
Go to 'Billing' and click on 'Manage PayPal':
Once you get the new screen, click on 'Create New Billing Agreement'
(if you already have one as "active", you can just cancel it):
Create New Billing Agreement:
After that you will be redirected to PayPal's website so you can provide your PayPal details, setup and authorize the automatic payments:
Once you've completed the steps on the PayPal side you'll be redirected to the client area again:
Automatic payments with PayPal are now enabled!