[trogan]

OK, I think I have this figured out now. The tl;dr is that I think I have the wiring correct as shown in the picture of my original post.

Here's the nitty gritty of why I think I have it correct. I had to get out the multimeter and check continuity between some things to make sense of it.

1. In the attached picture, all arrows of the same color are points which are common to one another (0Ω).

2. The power terminal at the bottom, marked with the orange and gold arrows, is tied to the +5v and GND pins on that big chip on the board. Based on that, I'm asserting that this should be fed by the PC +5v.

3. The power terminal at the right, marked with the purple and red arrows, is tied to +5v and GND pins that go with the IO pins of the breakout board itself. The jumper which decides whether your 4 axis COM pins (lines in blue) are GND or +5v is also tied to the same place. Based on that, I'm asserting that this should be fed by the machine's +5v power supply.

I feel pretty confident about it now, but if someone in the know disagrees, I'd love to hear. (Does Arturo Duncan from cnc4pc lurk in these forums?)