To use the screen sharing you must have:
1. The latest version of Java installed on your operating system
You can verify what version of java is currently installed on your system or download the java plug in here https://java.com/en/
2. Presenter status in the BigBlueButton meeting.
By default the first Moderator that enters the meeting is given presenter status; to pass presenter status click to the left of the users name.
If you are a viewer you will need to request presenter status from the moderator.
To share your screen:
Select the desktop share icon in the center of the screen:
Then select "start" from the presenter preview window:
Pressing start downloads screenshare.jnlp which, when opened, will launch an external Java applet that will share your desktop.
We also put together a quick YouTube Video showing how to share your screen on Windows and Mac OS: https://www.youtube.com/watch?v=xTFuEvmEqB0