I’m not sure about other Max versions but the file is there in 2008 and 2009. For every file in that folder, there should be one with a “_max” suffix: physics.mi, physics_max.mi.
I’m not sure that you can unhide it in the physics.mi file. At least mine doesn’t have the same code that you’ve posted. I don’t know too much about these files or their scripting language. I just know that you can unhide the shaders that are hidden by default.
As for how you unhide them, find the line that reads “hidden” and place a pound (#) in front of it. You’re just commenting out that line of code. If you can find another file that references the Physical Lens DOF shader, and it has a “hidden” value, try commenting it out and see what happens.