How to activate IPN in PayPal

After logging into your PayPal account click on the "Profile" tab and then click "Instant Payment Notification Preferences" as shown.

To change your preferences press the "Edit" button.

Check the activation check box, enter the script file's path as "www.mysite.com/cgi-bin/IPNSol.pl" (substitute your cgi-bin URL) and finally press the "Save" button to finish.

That's it.  IPN is now activated.