It comes with 3DMax and is really just a set of functions inside Max. You need a cd key to access it, so if it came with your version of max you shud have another key.
You access it by going to where max has the tabs Geometry, Shapes, Lights, etc. Its the one at the end call Systems. You create a Biped from there. The corresponding modifier is called Physique.
With Character studio you can rig with envelopes or by vertex or whatever really. By vertex is better for me, although you will have to go through some tutorials to get to grips with it. I personally have a book on it, thats how i learned.
Never really liked bones in Max, didnt get to grips with the heirachy of it.
Pete
I'm makin' a cake...