In order to set-up a recurring PayPal payment, you will need to access the invoice after it has been generated by logging into your billing area:
Go to Billing -> My Invoices to access them. Invoices are generated 7 days before the due date and you will have the option to set-up the subscription payment. You will also need to do this before the due date otherwise the option to set-up a subscription disappears after the due date on an invoice. You will need to follow the PayPal instructions on setting up the recurring payment to auto-process every month on the same date.