When you open your scenario using the greenfoot application you'll find a button share at the top right side of the application window. If you click this button you can uplad your scenario with a discription, an icon (screenshot) and tags. Also you can export it to a runnable jar file or to a web applet.

The 'Share' button near the top-right corner of the Greenfoot application frame initiates a dialog where the 'Publish' tab (at the top) is used to upload your scenario on the Greenfoot site. Just fill in the information, check the appropriate boxes and click on the 'Share' button.