Aldorei's Secret Part I

I'm stuck on the stage with the riddle and eating the cake 3 times and I see a button on the Eastern wall with the bookshelves and a sign that says "You will find the answer later in your journey" but no two pressure plates and I don't know how to initiate the redstone block sunflower wall puzzle.

Stuck on Stage 8, how do I start the puzzle? I don't see two pressure plates. I don't see a redstone block either.

 

Look for the section your stuck on in this guide. Hope this helps :D