Configuring Mail.app
Open Mail.app

Open Preferences

Select Accounts

Open new account by clicking the "+" button

Fill out account information

The Description can be whatever you want.
Write in your full name as you would like it to appear on outgoing corrispondence.
Incoming e-mail server should be calmail.berkeley.edu
User Name should be set to your account name on CalMail, if your CalMail username is the same as your asto one, then use it. Again, if it is different than your astro one, this is not a problem as the "Email address" setting above will determine what other people see as your address@astro.berkeley.edu.
Password is set to your CalMail password.
Select Outgoing server

Fill out Outgoing server info, calmail.berkeley.edu

Select the SSL check box, then select and pull down the Authentication drop down menu to "Password". Next, fill in your CalMail user id and CalMail password. Then click "OK".
Select Advanced

Select SSL

Local Folders or Archives
Remember, these folders will only be local to your Mac OS X machine. They will not be available on your Astro network connected machine unless you synchronize the files somehow.Select File and pull down to "Import Mailboxes..."

Select "Other" and click the button with the double right arrows

Then follow the navigation windows for finding your mbox files. I have declined putting up images for these as they use the generic file browsing windows that are standard across all Mac OS X applications and are self explanatory.
That's it! You can now use your address@astro.berkeley.edu using Mail.app.