ensure your java is path'd up correctly, and/or update/reinstall it.
It's a .jar - which win-rar can usually read and extract classes and such from it, but it should be runnable like a normal application, it has its own little java applet window for executing its functions with. - if its not running at double click, it likely means either that which I said above, or something else is forcibly taking .jar hooks, but I believe update/reinstall will still address that as well