If the divot doesn't line up, flip your RAM stick around.
Push your latches open, then look at each DIMM and position it over the slot such that the small divot on the bottom of the RAM stick is aligned with the matching bump on the board. If the divot doesn't line up, flip your RAM stick around. Insert the stick into the slot and push down on the DIMM on each edge until it snaps into place, causing the latches to close on their own.