How do I turn on the PayPal IPN and point it to the URL

I purchased this plugin for ‘fundraising’ for the honeymoon of a wedding website. Thus it is a list of things for the honeymoon and their prices. And guest should be able to contribute money towards this list.

The bride has a personal PayPal account.

Under settings, to set the payments up to her PayPal account there is the bit that says ‘In order for PayPal simple payments to log properly you must turn on PayPal Instant Payment Notifications and point it to… etc.’

Where/how do I do that?