Does this wiring look normal? If so, how can I connect a porch light to it?
There is no way to tell what to connect to. It depends on how they wired it. I think it is safe to assume you should connect to the single black since it is in that box by itself like that. And yes, you would connect to the white and green with the other two wires. The instructions are not sparse, they show what you need to connect to. They do not know or care how your house is wired, nor should they. I will say, that box for the porch light is NOT appropriate. That is a collar box meant to mount onto an existing flush box. The wood of the logs is exposed to the back side of the box. 

I also see a hole in the back of that collar box. Is this new light meant to be inside or out? If outside you'll need to replace that existing Wiremold collar box with a regular box, then run a cable from the back of the box into the back of a flush box outside.