I had a very similar problem, here is what I found
Both the ATX and ATX12V power connections must be connected to the MOBO. Unfortunately (at least for me), it took me sometime to realize that the power connectors that came with the tower are connected together. In other words I had to pull apart an 8 pin connector to make 2 4 pin connectors. They come apart very easily, but look like one 8 pin connector at a first glance.
So once pulled apart, I was able to plug into the 12 pin power connector (ATX) and the 4 pin power connector (ATX12V).
After that my machine would POST and give me beep codes.
Hope this post isnt too late and helps you. Not the most intuitive solution (again at least not for me, who is just getting back into PC building)