Hi, I am trying to set up an affiliate program and need to add code to my shopping cart and have been told I need to modify a template file called tpl_checkout_success_default.php. Does anyone know how and where I can do this? Both myself and the affiliate marketing co have been unable to find where to do this. Many, many thanks in advance!

You will never find that file

checkout_success.tpl.php in your templates content folder.. if it doesn;t exist there
templates/content/