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It has been a little while since I played Riven, but my two best guesses to this are A) The lights go on/off when you press the buttons on the bottom of the dial, or B) The book holding domes need to be stopped from spinning. I am downloading it now to check these situations and will report back when I have discovered something.
If I recall correctly the Whark comes when you press the button to turn on the red light.

Edit: I was correct. You press the outer tabs to turn the dial and press the pentagon button while it is in the red pentagon ring at the bottom to turn turn the light on. This is how the game gets the player to associate the symbols on the buttons to the colors.