nothing should be changed on the General or Database tabs. Only on the Session tab.
your set up is not quite right.
the roles table will define the roles that can be used, but you also need to add a roles column to your users table to set the role that the user belongs to.
the database tab should still have the users table selected.
the column in the general tab should be username and password, these should have the value set to come from the username and password form elements, these are the values used to see if the user exists in the users table. the database tab settings perform the login.
the session tab settings set session variables after the login is completed, only i the role column is a part of the users table can the role for the logged in user be set.