to tell why you are getting the error, I will need to see the code.
If you do not want a registration page, dont create it.
the simplest test you can run is to create only the login page using the wizard, then test it to see if you can login with one of the users you have already added to the database.
Do not do any additional modification from any of the tutorials, in your case, you are not using encryption, so the first tutorial does not apply. Dont worry about user levels until you can get a working login.
if a simple login page does not work, test the server for session management using the sessionTest.php file here: