It would appear that there must be some sort of knob on the bottom inner edge of the black shiny seat-back shell, onto which the holes in the "thingy" will engage. I suggest that you feel with your fingertips to try to locate any such knobs. If they are still there, then it is likely that the "thingy" needs to be pressed upward and inward until it slips over those retention knobs and they drop into those oddly-shaped holes.