If the System Preference is showing a Power Manager not running message, your Mac probably needs to be restarted.
On Mac OS X 10.4 Power Manager's daemon is controlled by launchd. This excellent tool lets you start and stop daemons from the command line.
To start Power Manager's daemon use the following command in the Terminal utility.
sudo launchctl load /Library/LaunchDaemons/uk.co.dssw.powermanager.plist
To stop Power Manager's daemon use this slightly different command.
sudo launchctl unload /Library/LaunchDaemons/uk.co.dssw.powermanager.plist