To run ModMultiSim under Windows, you can use the Windows Run dialog or double-click on the filename.

To run ModMultiSim under other operating systems (or from a Windows command window) use the command:

java -jar modmultisim3.06 .jar

You will, of course, need to use the full pathname for the modmultisim3.06 .jar file if it's not in your current directory. You will probably want to create a shortcut, launcher, shell script, or whatever to save you having to keep typing the above command.