Well at first I though that the barrel nut was the wrong way round but the screw thread is not in the middle so I think the plate bolts to the bench and you have to drill a hole for the nut. If there is no plate suppled then maybe you could make a plate to fit to the inside of the bench and attach it to the barrel nut. To be honest it looks like the steel plate will be sufficient. Also if you secure the barrel nut it will pull the screw towards the plate and make it bind.
I agree with steve two slots will need to be routed in the table to allow the box to slide.
Andy
Just re-read that - maybe a bit confusing :lol: