@ben2010uk, I think Oliver misunderstood your question. He gave you instructions for how to setup an IPN URL inside your PayPal account, but it seems you’re asking about where to find this URL in the first place..??
The answer to that is really based on what tools you’re using for IPN in general. You will need to setup your own IPN listener one way or another, and then when you have your own listener setup you can either add it to the PayPal account profile the way Oliver mentioned, or in the plugin settings which is what I think you were referring to.
If you need help setting up an IPN listener I would recommend checking out our free PayPal IPN for WordPress plugin. This will get you up-and-running very quickly with an IPN listener inside WordPress. The plugin provides a bunch of hooks you can use to trigger your own functions based on PayPal transaction type or status.
Let me know if you have any questions or concerns about any of that. Thanks!