Slamdot gives you access to two web mail applications: RoundCube and SquirrelMail.
The default is RoundCube, accessible at http://www.yourdomain.com/webmail.
If you prefer SquirrelMail, simply go to http://www.yourdomain.com/squirrelmail.
To use either of these web mail applications, just login with your email address - this is the username - and your password.
You can also access web mail from the control panel. From the dashboard, choose a plan. Click the email tab and click the users link. There is a button to the right for accessing the web mail login page.
Note: All web mail applications make use of the IMAP email protocol. Check out How do I configure and use IMAP with Slamdot? to learn more.