Put in http://webmail.thenameofyourdomain.whateverextension and log in with your email address and your password.
Example: If your domain was kyeyes.org then your webmail address to login would be http://webmail.kyeyes.org. It is just that simple. P.S. Be sure to bookmark the webmail server before you login so you go there each time.

The new page now says welcome to Horde Mail. There are lots of new features and changes with webmail so take a look around once you log in.