http://www.microsoft.com/mac/downloads.mspx?pid=Mactopia_AddTools&fid=31BC88ED-DF2D-471A-B4B5-66C2A51C8BEAScroll down and look at the description on the lower right side of the screen.
There is probably a "search for updates" selection under the Help menu. They may also have a setting in Preferences for notifying you when updates are available.