I've tried pushing it in and it just simply won't hook into the motherboard. I can't start up the computer and would really like to have this fixed. I'm using the Corsair CX 500 PSU and z87-a motherboard.
More aboutpin connecting motherboard
Make sure you are hooking it in the right direction until you hear a click. It takes a little amount of force in case of a new build.
Do you have the clip lined up, to pop over the tab.