First you have to join the jam, using the big red "Join jam" button. Then when you're ready, go to your dashboard and click "Create new project". Fill in the details. Once you're satisfied (game is uploaded and so on), set visibility to public at the very bottom and save. Back on the jam page, if I remember correctly, there should now be a "Sumbit game" button. From there you can pick the game you just published, and you're all set.
Oh, sorry, I find out Publish button and now everything is submited, as I think :)