You can access webmail in any of the following ways:
http://yourdomain.com/webmail/
http://yourdomain.com:2095
https://yourdomain.com:2096 (secure)
Note: replace 'yourdomain.com' with your actual domain name.
You will be prompted with a user/pass dialog box. for the username you will use the full email address and the appropriate password. example: user: you@yourdomain.com pass: yourpassword
You will then see a page with 3 options to choose your webmail client. Horde, roundcube or Squirrel Mail.
You also have the options: [Change Password] [Forwarding Options] [Autoresponder Options] you can modify those as you please.
Ports 2095 and 2096 must be available to use webmail. If you are behind a firewall such as at work, webmail may not be available unless they open those ports.