PayPal IPN Activation Instructions
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" and finally press the "Save" button to finish.

That's it. IPN is now activated.