Press the button for starting the editor. Download of the editor class files will take some time (998915 bytes), but then a new editor window will show up on your screen, if you have installed the Java 2 plugin (Sorry about that, but we make heavy use of Java 2 features...)
If you have granted read access permissions to your Java plugin (see the Java Tutorial for details), you can load a sample statechart into the tool and simulate it. Please make sure that you put the downloaded file into the directory to which you have granted read access. The user interface of the applet version of the statechart editor for selecting files is very simple due to general access restrictions imposed on applets. You have to type in the complete path to your downloaded file (sorry...). If you, furthermore, have granted write access, you can even save edited statecharts and load them again.