I see a lot of info on login and security info but cannot find any on creating a full user registration with Database updater 2, DMXzone Mailer and Security Provider 2. is there any manual or tutorial on this?, and if not can your point me on how to? I can create it from a inset form and maybe figure out how send the email using the form to mail, but totally lost on how to check if the username already exists.

There is no such a tutorial available. To check if the user already exists, just query the users table and see if the query returns results or not.