Because of a few cancelled orders there are still around 30 “last minute” tickets available at the entrance. You can buy such a ticket through the “service lane” at the entrance of FoM 16.1 for the price of 50 EUR cash.
You will then be able to take one of the white or hatched (locked) seats on the seatmap:
http://www.fom.be/seatmaps/view
If you still have unassigned tickets, or didn’t reserve a spot on the seatmap yet, then you will have to do this before you come to FoM or you will have to go through the “service lane” before you can use your ticket.