That button (above) opens 'Hangouts on Air' window (pictured below). Next click 'Start Broadcast'
12a) You can't make Keynote (i) full screen and (ii) you can't hit the "play button" to start the slideshow / presentation.
You can't do either of those, because it will cause Keynote to not appear in the 'Hangouts screen selector window', thus you won't be able to broadcast the Keynote presentation through Hangouts.
To make it work, make Keynote look like it does in the screenshot below.
And start scrolling up and down along with David's talk, with the keyboard shortcuts mentioned above; do not try and use the mouse, it will cause problems, use the keyboard shortcuts.