If it not on your dock (icon looke like a stamp) you can also go to your applications folder/ Mail .ap andd double click it, that should open the Mail application. You can also drag the icon from the appliactions folder to the dock and drop it there to create a short cut. If you have not used Mail before ,you may have to set it up to work with your internet mail.
It should prompt you thru it, but if not, you can check here for some help:
Also check this:
If still no joy contact your provider, and they should give you the info that you need to enter to have it work on their system.
Hope this helps