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 plugin 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 user's 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