PayPal 
1Log in to your PayPal account
2Click on the Profile subtab
3Click on the Instant Payment Notification Preferences link in the 'Selling Preferences' column
4Click Edit
You should now be presented with a screen containing the following:

Setting callback URL

Note:
If the Instant Payment Notification link is not present you will need to apply for Premier Account or a Business Account from PayPal.
Check the checkbox to activate Instant Payment Notification and enter the following URL below the checkbox.

https://www.example.com/forums/payment_gateway.php?method=paypal

Click [Save]

Go to Paid Subscriptions > Payment API Manager > PayPal and enter your PayPal email address in the PayPal email field.
ConqSoft 31st Dec 2005, 12:55am
The PayPal IPN no longer needs to be set, since it's sent with each payment. This section of the manual needs to be changed/removed.
Chris 06th Nov 2012, 02:21pm
The Paypal IPN may not be required for vB, but Paypal requires you to enter a URL in order to have IPN active. So you must enter something in the URL field.