Need to add CURL code to IPN Process script

Well when the payment is done via PayPal then PayPal sends instant payment notification to selected script with information about the order. That is the notify_url so PayPal is sending the notifications to

I need to know where this script is located so I can add a specific CURL Code -so that the code captures the pap_custom URL parameter from the notify URL and resends the notification to the Post Affiliate Pro's PayPal plugin.

The point of the integration with PAP is to make sure that the cookie information is attached to this notification call, that is done by the javascript I already have installed in my wordpress.

Can someone tell me where this membership 2 Paypal Standard IPN Process script is located?