Layer help! Not sure what I'm doing wrong.

I downloaded this brochure mockup today - but I can't seem to figure out why the turning page isn't showing the design layer. As you can see from the layers panel, I have imported both designs into layers A and D. However B and C are not smart object layers. I would like the middle turning page to have a different design on either side, but I can't work out how to do this! I'm getting really frustrated with myself!
Also, what does 'click & update' on the two green layers? Do I need to edit those layers so show designs (how?).
Thank you
Kelly
Running MacBook Pro 2012 Version 10.13.2
Adobe Creative Suite CS6 Photoshop