how do I set up the Moneris mpgClasses.php file to work with my wordpress install
I haven't used it myself, but a quick search pulled up this extension by Alan Dixon and this related post: Does the core Moneris payment processor work?

Also, have you looked at the documentation?


Setting up Moneris Hosted Payment Processor for CiviCRM
Moneris Configuration Guide


I hope that helps.