Lets start at www.paypal.com
Go ahead and login and follow me.
Lets start by hitting the profile button on the right hand side
once there lets go to the email link in the first column
there you will see this
Make sure that the email address here matches what you have in your Zen Cart settings ( Case Sensitive )
Now lets go back to profiles and hit the Instant Payment Notification Preferences link in the third column.
Hit the edit button
make sure that the checkmark to use IPN is there and type the correct link in the text bar.
http(s)://(YOUR STORE)/ipn_main_handler.php
Hit Save
Next in Profiles hit the website payment preferences.
Make sure that auto return is turned on
then get the return address from the paypal module in zencart admin.....
Make sure it matches EXACTLY.
you can just leave the other settings as they are ( Exception the Paypal Account Optional, this is your personal preference )
Save
Go to Zencart and create a $0.01 item marked as free shipping and use a personal account or a friend to test for you....
If you have any Troubles look in the trouble shooting guide.