Because RT System Editor is an Eclipse plugin, it first requires that Eclipse and the other Eclipse plugins it depends on be installed. See the installation instructions for details on how to do this. Then copy the RT System Editor jar file (jp.go.aist.rtm.X.X.X.jar) into the eclipse/plugins directory as described in that document.
The first time Eclipse is started after installing, the Welcome page will be displayed.
Click the cross in the top left of the Welcome page to close it and display the below screen.
Click the Open Perspective button in the top right and select Other...
Select RT System Editor to start the plugin.