To start using Paypal Express Checkout as one of your payment gateway on your eDirectory, you have to get Paypal Express Checkout on Paypal's website. You can get it at the following link:

See the images below showing where to create your account.

After creating your account, with all the information necessary, you will be able to integrate with your eDirectory. eDirectory system request the API Username, password, and the Signature that Paypal gives.

Here is how to get the signature.

Log in to your account, and click on Account Settings

Click on Account Access and then the link next to API Access.

Click on Manage API Credentials under "NVP/SOAP API integration (Classic)"

Copy this information, they are necessary for the next step.

Now go to your sitemgr. Go to Configurations, Manage Levels & Pricing, Payment Gateway.

Enable the Paypal Express Checkout, and complete with the information you copied before. After that, save changes at the end of the page. 

Now your Paypal is set and ready to use if everything was completed right.