PayPal Payments Pro
PayPal Payments Pro, also referred to as Website Payments Pro sometimes, is the Merchant Gateway Solution offering from PayPal. It allows you to process payments direct from clients credit/debit cards to your PayPal account without the customer ever leaving your site.
Configuration
To activate the module, begin by going to Setup > Payment Gateways and choosing "PayPal Website Payments Pro" from the available list of gateways to activate.
Once it's been added, you will then be asked for your API Username, Password & Signature. These are obtained inside your PayPal account and we have a guide showing exactly how to do this @ http://wiki.whmcs.com/PayPal#Automated_Refunds
3D Secure
The next 3 fields: Processor ID, Merchant ID & Transaction PW are used by the 3D Secure process. PayPal Pro Accounts are usually enrolled for this by default these days and if you've been enrolled, PayPal will have provided you with the details for it.
If they haven't or you don't want to use the 3D Secure feature, then we have an older version of the PayPal Pro module that won't perform attempt to perform 3D Secure at checkout and so doesn't have these fields which you can obtain @ http://www.whmcs.com/members/dl.php?type=d&id=58 and then just upload to the /modules/gateways/ folder of your installation to have take effect.