Well, a 'main' class is obviously, the main one. You don't see it in greenfoot because of the World-Actor stuff, but I think it is implicit and it's the one that contains the "main" method. For example:

I would just try different things to see if it works. Maybe start with the path to the folder the project is in and then path to the jar file itself from there. Maybe it might work -- maybe not. I would think the jar file would contain a class (presumably your initial world class) with a 'public static void main(String args)' method already created within it. You may also have to include a path to the classes that greenfoot supplies. The link I gave above shows an example of multiple paths for the first part of the command line. Like I said, try different things -- you might get lucky.