What modifications to my DNS do I have to change to reroute mail to Postini?
Delete your Current MX Records and replace them with the following Postini Mail Servers:
domain.com.s5a1.psmtp.com. [Pref = 10]
domain.com.s5a2.psmtp.com. [Pref = 20]
domain.com.s5b1.psmtp.com. [Pref = 30]
domain.com.s5b2.psmtp.com. [Pref = 40]

Where do I find help with the Postini Message center?
A PDF that can be emailed to the end users can be found here.
Message Center Help

Where do I find help with the Postni System Administration Console?
Here is a link for System Administrators to find more about the Postini Message Center.
Admin Console Manual