The information is on your DT Register configuration screen.
"You must have Instant Payment Notification (IPN) enabled in your PayPal account for registration records to be created properly in DT Register. To do this, login to your PayPal account and go to Edit Profile. Under the "Selling Preferences" column, click on "Instant Payment Notification Preferences". The next screen will show if this is On or Off. To turn it On, click the EDIT button. Then check the checkbox and then input a url in the Notification URL box. DT Register actually has the necessary URL coded in, so it doesn't matter what url is input here. If nothing is there, just type in your website URL including the http:// on the front. Click SAVE and you're all set!"