Did you set the variable:
Code:
auto_create_user = TRUE;
in /var/lib/roundcube/config/main.inc.php ?
Are you sure that you set up correctly the mail server?
The new box that appeared, gives you the ability to choose the mail server to login. If the mail server is local (as the manual assumes) then the localhost is correct.
Please verify that you set the auto_create_user to TRUE