Hello,
Seems like you went to the IPN testing page, not to the IPN "turn on" page.
go to sandbox.paypal.com, login with your sandbox account
then look for
My Selling Tools -> Instant Payment Notifications
That has to be the proper location, there is a form where you need to input a url:
yourdomain.com
and select the IPN status as enabled.
Let me know if you need I'll try to make a video tutorial if you can't find it.