In Store -> Payment -> PayPal Express, you could enable PayPal Express checkout along with your credit card payment. Before you configure PayPal Express, make sure you have signed up PayPal Payment Pro account. To enable "PayPal Express", click "Enabled" checkbox. Enter other neccessary fields that are required to complete PayPal Express payment module. Here are some field explanations.

• Use Sandbox: when you enable this, all the transactions are redirected to PayPal Sandbox which is a testing environment. If you want to test PayPal Express checkout model, you could enable this. Be sure to turn it off before go live.
• API Account Name and Password: login to your PayPal account. In Profile -> Account Information -> API Access -> enable API Certificate. They you will be given a API Username and password. Check https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/apicertificates for the nice instructions from PayPal. You need to follow only Step 1: Generate Certificate.
• Certificate and Password: after you have done the above step, you could download a certificate. This certificate is in PEM file. You need to convert it to P12 file. Check https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/apicertificates for Step 2: Encrypt Your Certificate  to convert your downloaded PEM file into a P12 file. If you need us to help you convert the file, please send your PEM file to us. After you get the P12 certificate file upload your P12 certificate and password here.

Do not forget to click "Save" button when you are done.