There must be hundreds of "black metal extendable dining table". It would help IMMEASURABLY to know which one you have. IMMEASURABLY.
Why not contact the retailer?
My best guess is that you grab hold of the frame at one end, pull it to slide out a bit, along with half the top and drop the centre section in.
This might help. There's a 360 degree viewer that shows enough detail to work out what happens for an Ikea item