you use mysql backend. you just need login the postfixadmin, manager the domain, add the user.

no need modify any files.

I not know which version your install , mysql or ldap

mysql
you can use postfixadmin manage the domain
https://ip/postfixadmin

ldap
you can use phpldapadmin manager the domain

https://ip/ldap

hope you can read the iredmail.tip /root/iredmail/iredmail.tips
would solve all your problem.

I am using the mysql backend.

I did read the tips section but it does not tell me what I have to do. This is a page from ROUNDCUBE webmail. This is what I have to do:
http://trac.roundcube.net/wiki/Howto_Config

When I loggon as postfix admin I do not see how to what what the wiki says. I had to manually edit files.

I have found that I need to put  $rcmail_config['default_host'] = 'machine';
I have modified entry in /var/www/roundcubemail/main.inc.php and main.inc.php.dist but that does not seem to work.

What should I do?

mysql
you can use postfixadmin manage the domain
https://ip/postfixadmin

ldap
you can use phpldapadmin manager the domain

https://ip/ldap

hope you can read the iredmail.tip /root/iredmail/iredmail.tips
would solve all your problem.

I have successfully installed your iRedOs. I can access the roundcube webpage but how do I set the imap server that roundcube goes to for mail? How do I loggin as the administrator?