It is a bit tricky... I think the idea is to put the screen flat on top of the rest of the Pi-Top and then slide the two rectangular metal hinges of the screen into the slots of the base. They can then be screwed into place with the two tiny screws. Is that what you are doing? What doesn't work is to try to get the hinges in place when the screen is in the open position. I hope this helps.
I had a similar problem. I seem to remember that one of the metal "C" shaped pieces in the hinge needed a gentle widening by inserting a flat bladed screwdriver and prising it open a bit.
Careful though, it is made of brittle metal.
It solved it for me however.
Absolutely cannot get left hinge inserted. Right one goes in with significant force. Left side takes more force than I can accurately administer. I have not YET accidentally shot the laptop across the room nor broken the screen. Though the bevel is starting to delaminate.
What Jez said, i also had to widen the right side metal slot with a screwdriver to get the screen hinge to fit into the slot, spread the metal a bit and it slid in ok.
good luck, Rick