you probably mixed user and client.
To manage websites you have to set a client who will be able to manage his websites.
For access to client's website control panel use https://domain.com:81
(if you have SSL website enabled) or http://domain.com:81
(if you don't).
Client have login data set on 'login data' tab - there you can set for your client his/her username (to be used for login instead yours 'admin' username) and his/her password.
Remember that if you create on 'new site' a user under 'user & email' tab, this user is able to access his/her email only. S/he is not able to manage websites nor email forwarding etc.
Client is able to log onto website administration and do settings for email user under ISP manager => website (domain.com) => user & email tab => user (unco) => advanced settings tab => forward.
hope this help