Maybe it's a problem of Java version (this editor requires Java 1.5 or later).
Anyway, I don't know how Java error messages are shown on Mac, but you could try running the editor from a terminal to see if more specific messages are shown that way. To do that, open Terminal (under Applications - Utilities), change to the directory where the .jar file is (for example, if the .jar is in the Downloads folder, type 'cd Downloads') and type 'java -jar KSEditor.jar'.