This part is a bit tricky, because you need to connect the mainboard onto PCI riser card as you slide it in.

Begin by placing the mainboard connectors onto the I/O shield at an angle, as shown in the first photo.

Gently push the left edge of the board and verify that the riser card is properly mating with the PCI slot, indicated by green arrows in the second photo.

If everything looks good, push the mainboard all the way onto the hexagonal mounting posts and secure it in place with screws