In the top picture, you notice how the bottom connector on the right hand side of the box looks like a printer cable? The breakout board is on the inside of the scanner and is what that plugs into. The printer cable leads away from the scanner to the DAC, which is in turn connected to a PC via USB.
Alright, here's some common sense stuff. If you've got an electronic device that doesn't use batteries and it has two cables coming out of it then one of them has to be power, right? The other connector above the printer cable is the power.
The big black box in the bottom picture is a computer power supply that Andy put in there to power all the fans, most likely. The other thin black box is the laser power supply, which is where the modulation goes.
No offense was meant in my last post, but the thing is you have to do some research and thinking on your own. I can't spell every last detail out to you because then you wouldn't be building your DIY scanner, you'd be building my DIY scanner.
Like I said before I have absolutely no problem helping people, but I much prefer it if you do some research on your own. You'll find out much more that I can tell you and on your own terms, so it'll stick better.