Try changing to ownership of the .plist file, as noted here.
sudo chown root /Library/LaunchDaemons/myfile.plistsudo chgrp wheel /Library/LaunchDaemons/myfile.plist
It is also worth noting that these root LaunchDaemons can't be world writable, for security reasons:
sudo chmod o-w /Library/LaunchDaemons/*
Try changing to ownership of the .plist file, as noted here.