There are several ways in which you can make them do that.
1. Their profile page
The user profile page is the easiest place in which they can set their PayPal email address, just as any other profile settings. The feature is enabled by default and you need to take no action. However, this requires that authors have access to wp-admin, at least for profile editing, which they may not depending on your setup.
2. Any public page of your choice
The PRO version comes with several shortcodes for stats. It also ships a shortcode to display the PayPal email address form in any public page of your choice. You just need to create a new page, enter [ppc_user_paypal_email_settings] as text and publish it. Logged-in users visiting that page will see the apt field to enter and change their PayPal email address.