The Thin Mini-ITX power issues have been pretty well covered in that thread, as well as by [USER=17]@QinX[/USER] with two of his builds.
Basically, your board is powered by a 19V AC adapter, while you need 12V for your GPU and powered riser.
The HDPLEX board sole purpose is to provide that 12V voltage.
To provide power to the HDPLEX board, you can use the motherboard 2-Pin connector, used as passthrough output, your AC Adapter being plugged into the motherboard DC connector.
However, for the HDPLEX to start, you need either to jumper its 24-Pin connector (quick solution), or use something like the Add2PSU (what you call the green PCB) or any other circuit to allow the motherboard to start the HDPLEX on startup (elegant solution).
It works great for me 
(case and cable management are temporary).
