The web interface for your website mail is located at:

http://yoursite.com/roundcube

To log in you need to use the email (type user@yoursite.com) and password created in the control panel ISPmanager, in the E-Mail -> Mailboxes section. You can also log into the web interface for mail through the ISPmanager control panel in the "Mailboxes" section -> select a mailbox -> "Login" button.