How do I re-connect this Ribbon to the Circuit Board Connector?

I have successfully repaired my Nikon D600 camera using ifixit info, except that the 'thumb-wheel' under the top display does not work.

When I took off the top of the camera, I pulled out the ribbon from the circuit board that is immediately below this 'wheel'.

During re-assembly I could not work out how to open this connector, so I pushed the ribbon back into place with difficulty. It might not be fully home.

The attached 3 images are, I hope, self-explanatory.

How do I get the ribbon properly engaged?

Or, just possibly, what other component have I not re-assembled properly?

Thanks!

Looking forward to taking it all apart again ...... and getting a perfect fix !!