For Express Checkout payments a third party library (JMSPaymentPaypalBundle) is necessary. For installation and setup please look at: http://jmspaymentpaypalbundle.readthedocs.io/en/stable/setup.html
Please see How to get the API credentials to learn were the values for the JMSPaymentPaypalBundle configuration can be found in the PayPal merchant's administration.
Additionally, the SisoPaypalPaymentBundle must be activated in the Kernel and the routes must be included:
The configuration for payPal Express has to be setup for each siteaccess (at least when the backend is used).
Login and navigate to "All Tools"
Select the tool "API Access"
Choose "Classic (NVP-)API integration"
This page will list the necessary values.