You can access webmail in any of the following ways:
http://yourdomain.com/webmail/
http://yourdomain.com:2095
https://yourdomain.com:2096 (secure)
replace yourdomain.com with the name of your website
You will be prompted with a user/pass dialog box.
For the username you will use the full email address and the appropriate password for that email account.
example:
user: you@yourdomain.com
pass: yourpassword
You will then see a page with 3 options to choose your webmail client.Horde, Squirrel Mail, RoundCube.