Well, you have to have the bones named exactly right....well ok maybe not but it does make characterizing in motion builder alot easier. you will also find that the rig requirement's for motion builder are a little different to the norm, two thigh bones for one and two fore-arm bones. if you go through the rather laborious MB help you'll find the correct naming convention.
then it's a simple task of, select character, hit characterise, and load up whatever motion you want. i can't remember how many come with motion builder though, i seem to have picked up a few over the years but it's questionable how much the "man wiping feet" motion is.