the Mail icon in the DOCK will launch the Apple Mail application and only requires you to enter the details of the email setup you have with your Internet Service Provider.
It will either be POP3 or IMAP and you will need the server names, user names and passwords to configure it.
The MAIL application is not just for the Mobile.me/Mac.com service, it is just a regular email client.
Hope that helps
i don't have mail @mac.com. how can i still use the icon in the tool bar to access my other e-mail account?