WHMCS and Pro Sites Payments

Hi I have a question regarding Pro Sites and WHMCS.

I have WHMCS now installed on a separate domain from Pro Sites.

I am trying to activate PayPal payment in WHMCS but am having trouble because in the directions it instructs you to create an IPN in relation to your WHMCS installation.

Pro Sites additionally requires an IPN established in PayPal.

Has anyone dealt with this issue? If so, what did you do to get this to work correctly?