Use of IPN with thecartpress ecommerce site

I am setting up an ecommerce site using thecartpress plugin.

I have read and read instructions about IPN and the IPN listener but am still very confused about the whole thing.

If I tick Log IPN data within thecartpress do I then need to give Paypal an IPN handler URL?

And if I do need to do so what is that URL? Presumably it is not just the url of the site I am working on? I tried using just that url within the IPN simulator and when I checked the IPN history nothing had been sent.

Sorry if my questions are confused but I found the whole IPN issue overwhelming. I think I have everything else working but this has really stumped me.

Any assistance would be very much appreciated.