first off, the IPN page is not a page your customer will ever visit. Paypal will send a post to this page behind the scenes.
I recommend seeing this page for details and sample code.
when setting up the IPN page, open the bindings panel, click the plus button and select eCart -> Checkout -> Checkout Form Response. from the gateway selection list, select Paypal IPN Cart. this will add bindings fro the IPN post to your bindings panel to use.