For a detailed description see http://kde-look.org/content/show.php/Service+Monitor?content=125203
Simply run the following command:
$ plasmapkg -i /path/to/service-monitor
then add the widget in KDE. If you want to remove it again, run
$ plasmapkg -r service-monitor
For development, you can use plasmoidviewer to quickly start the widget from command line. This way you don't have to install/uninstall all the time.
Please note: the master branch may contain a temporarily broken version; switch to a release branch for a working version.