It's not kicking off, it reports "Invalid property list".
it's in /Library/LaunchDaemons and should start with sudo launchctl load /Library/LaunchDaemons/YourMac.name.here.SftwrUpdt.plist
I finally got tired and set it aside for now.
I've scrutinized it for syntax errors and searched for the reason that it's invalid.
Different versions of OS X use different file locations and the finished (working) plist might be version dependant, idk.
the "softwareupdate" command works from terminal so thus far I just haven't figured it out.
I'll have to go back and study Creating Launch Daemons and Agents to see what I might be missing.
I was going to use cron but since it's depreciated and I wanted to help as many ppl as possible I went with Launchd.
Just a work in progress - I thought I would share with you.
Don :)