Where is the ground wire on this light fixture controlled by a motion detector?
It looks like the fixture is plastic, so it's not likely you'll have to ground it.  The fixture strap (the brass color metal bits in the bag below the fixture in the photo) should have a threaded hole, where a green grounding screw can thread in. 

Install a grounding screw in the threaded hole of the fixture strap, and attach the house grounding conductor to it.  

I'm guessing the instructions included with the fixture are boilerplate documents, and are included with a variety of models.