if you're still using the Z77 Extreme4-M and the Seasonic x-400, it appears your motherboard has an 8 pin ATX12V1 connector (some mobos only hav a 4 pin connector) and the psu has one modular cord with a 2x4 pin connector that may or may not click together but in any event, connect those two 4 pin connectors to the 8 pin plug on the side of the motherboard next to the black heatsink marked Z77 (item 4 on your schematic in the manual).