Many years ago, we created the ability for users to upload Greenfoot scenarios to our website, and play them from the website without requiring Greenfoot to be installed locally. This was a great selling point. If you were a learner creating games at home or at school, it allowed you to share the game with friends and family by just giving them a link to the online game.
The Marbles scenario is a good one to try. Remember that you need to press the Run button before playing. (Enlarging this button and making it easier to start playing is one of the items still on our todo list.). Then drag from the gold ball to fire it.