Log in to your PayPal account.
Instant Payment Notifications
Go to Profile -> My Selling Tools -> Instant Payment Notifications
To enable IPN, click on this radio button: Receive IPN messages (Enabled)
You have to enter a Notification URL. It can have this form:
Auto Return for Website Payments
Go to Profile -> My Selling Tools -> Update Website Preferences
Set Auto Return to On.
You can enter a return URL below but it is also sent from your shop.
Related Help Screen.