You can do one of two things:

1) Go to http://www.jempeg.org/installers-20b1/ and download the original installer for Windows, then download the jar file from the link at the top of this thread and replace the jar file in the install directory (don't unjar it, just replace the .jar file)
2) If you already have java installed somewhere else, you should be able to double click on the jar file, or run "java -jar jemplode20.jar". If you don't have java installed, then follow the instructions in #1 to download an installer that comes with Java.

Mike