This tool requires Java Runtime (JRE) 1.4.2 or greater. You may already have this installed. From a command prompt type "java -version", and if the result is "java 1.4" or higher, you should be set.
Currently this can be retrieved at the following URL if you do not already have it installed.
The full development environment is available at http://java.sun.com/j2se/1.4.2/download.html.
Supposedly, just the basic java runtime can be retrieved at http://java.sun.com/getjava/
Once java is installed, you should be able to launch this program with "java -jar mm8leveleditor.jar", or if you're using windows, you can probably just double-click on the mm8leveleditor.jar file.
Technical note: The jar file itself contains the classpath information needed so that it knows to look for unlod-base.jar and jmf.jar (only used for wav file support) in the same folder as mm8leveleditor.jar. If you choose to run this using an alternative method, note that all three jar files (mm8leveleditor.jar, unlod-base.jar, jmf.jar) need to be in your classpath, and the Class to execute is "org.gamenet.application.mm8leveleditor.MM8LevelEditor"
You might need to grant java more memory (java defaults to 64Mb total) for large data resources. Use "java -Xms1024M -Xmx1024M -jar mm8leveleditor.jar" to do this.
The preferences for this application are stored in a file called "MM8LevelEditor.properties" which will probably be created in your current working directory. Some future change may make the location of this file easier to deal with. For now, you should try to keep your working directory in the same place as your jar files. If MM8LevelEditor is unable to find this file, it will create a new one.