By default, participants have not the right to start a presentation, they need to ask the host of the meeting to enable this option. The following messages will appear, in the top right corner: Unable to render embedded object: File (escritorio.jpg) not found.
and on the bottom right corner:
Click on Start in any of the messages displayed to start sharing. A pop up window will display the options to share windows, applications, or your entire desktop. The windows or applications to share must be already opened in order to be selected in the list of applications to share.
Once you have selected them, keep the applications maximized to provide a better view. If you minimize them, the participants will not be able to see them.
To end the presentation, you need to click on Stop Sharing (using any of the buttons available) and go back to the main configuration screen Share My Screen to start the sharing process again.