In order to share your desktop via WebRTC client in the Chrome browser and add the required Chrome Extension prior to your call.
When you connect to a conference (follow the provided link and open in your Chrome browser, then select [Computer] > 'Using your browser' and then [Get Started].
To the right of the video window you should see Screen Sharing and "Your browser is supported, but you have to enable screen sharing."; click on the hyper link text 'enable screen sharing' to be brought to the Chrome Web Store to add 'My Screen Sharing' to Chrome (click on the blue 'Add To Chrome' box in the upper right of the popup) and confirm by clicking the [Add extension] button. Close the Chrome Web Store and return to the connection page which should have a popup message asking that you refresh your page (you can click on the hyperlink in the message to do the refresh).
Now just connect again: [Computer] > 'Using your browser' and then [Get Started] and once you join the conference you will find an additional button on the floating onscreen menu that looks like a computer screen. When you click it you will be able to select what window or computer screen you would like to share and then push the [Share] button.
To stop sharing push the computer button on the floating menu again.