X-Cart:Setting up Your API Access
This step is required only if you are setting up PayPal Payments Pro without a PayPal Manager account or if you are setting up Express Checkout 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.