IPN NOTIFICATION HOW TO?

[Remi]

I have installed and test successfully payment using paypal´s sandbox for standard Checkout.
IPN notification is checked at paypal, but I can not figure how it works.
I have configured a return url to go back to the site after payment, but paypal redirect to the cart with a parameter:
"http://www.examplesite.com/checkout.html?uid=4e6bb57e1e3db5.31121204"

I see in the form before sending payment the path for "notify_url":
"http://www.examplesite.com/system/modules/isotope/postsale.php?mod=pay&id=2"

Can anybody explain me how to set this.

Thanks

[desperados]

I've installed Pay Pal Standard payment too, and I don't understand how it works
in fact, there are no field where insert return url
and when in frontend I make an order, it give me a blank page: where can I configure the paypal url?