I doubt it's in the instructions. It's in the coolant pipe that runs from the engine (out) to the heater core (in). You have to cut the exact same pipe to add a coolant filter, so it's the pipe shown in this thread (picture 2 & 4).
The restrictor is just an aluminum tube placed inside the rubber pipe, so you should be able to squeeze the pipe with your hand, if you can't, don't cut it at that point.
Again not all 6.0's have it, as it was a recall / update, and I *THINK* if you do you will see a pipeclamp around the hose to hold the restrictor in place.