How to schedule Mac software updates at a particular time in Yosemite?
How are you searching for it?  If in Spotlight, don't. The Software Update.app bundle is exactly where the reference states that is it!  Its fully qualified pathname is /System/Library/CoreServices/Software Update.app and you can see it in Finder by starting in the root of the Macintosh HD or if in iCal when selecting the file to open, make sure you start navigating to it from the root of the Macintosh HD (or whatever the name of your HD is).