Accessing webmail is fairly simple. As far as your domain name is pointing towards the StableHost DNS, you can point your browser at http://yourdomain.com/webmail
For example, if your domain is bobsfish.com, you would go to bobsfish.com/webmail
If however your domain name is not pointing to the StableHost DNS, in your welcome email that you have automatically received from the system, there will be a server URL where you can access the webmail directly.
If you are using it correctly, you will see the webmail login page, where you can login with your email address and password which you've setup already in cPanel.