Go to JGF (bottom of my post), go to the articles, and read the article by Kevin Glass on Webstart/JNLP. It’s short, clear, and tells you how to make webstarted games.
Webstart is a way of running java games on a PC/Mac/etc just by clicking on a link.
I doubt you want to package your mobile game using webstart, unless you’re intending it to be played on PC’s with an emulator (which you would have to distribute with the game, all wrapped up using webstart). Instead, I think the contest is probably perfectly happy for you to just provide a MIDlet or etc, but they didn’t think much about the rules when writing them and made quite a few mistakes.