This has nothing to do with the motherboard. It is totally dependent on which chassis or Pc case you get.
The important thing is that you fix the 'riser' to the correct position that match the holes on the board.
You may wish to contact the manufacturer of your chassis if you are not sure where the plastic spacers goes onto the case. They are the only ones that will be able to give you the exact steps and guides.
I have found a short video on you tube which explain briefly how to integrate a motherboard into a pc case.
Hope this will be helpful.
That's probably a good idea to notify Intel. I think with most motherboards, there is info about what's in the box. I searched for it, and like you, I came up empty. That's a dirty trick to put something in the box and not tell you about it.
It's my experience that the motherboard spacers come with the case, not the motherboard. I think usually the motherboard comes with a CD, some instructions, and maybe a few cables. Those spacers might be there for a reason, but Intel should tell you what the reason is.
By the way, not all motherboard spacers are plastic. I use the Antec Super Lanboy cases and they come with brass spacers that screw into threaded holes in the plate that holds the motherboard.