X-Cart:Setting up Your API Access
This step is required for Website Payments Pro and Express Checkout used as an additional payment option.
To enable your X-Cart based store to communicate with PayPal servers, you need to obtain your API credentials from PayPal. API credentials identify you when you make API calls. PayPal's API credentials consist of an API username and password (these are not the same as the username and password you use to log in to paypal.com), and a third piece of information - either an API signature or an API SSL client-side certificate.
More information about PayPal's API credentials and instructions for requesting such credentials for your account are available here: http://qwyx.co/apihowto.
Important: Please note that to be able to use an API SSL client-side certificate as your credential, you need to be able to install the certificate on the web server in the directory <xcart_dir>/payment/certs/, where <xcart_dir> is the directory where your X-Cart is installed.