When you have a scenario open in Greenfoot, there is a 'Share' button in the upper right corner of the frame. Click on it and select the Publish tab along the top. Fill in the appropriate information, select publish source code if you like, enter your Greenfoot password and press 'Publish' or 'Upload' or 'Update' (whatever is on the button toward the lower right side of the frame). Your scenario will be put on the Greenfoot website for all the world to see.