While working on the "Omiqron" Project I needed to go from Max to Motionbuilder to animate because of the freedom there is in Motionbuilder and the possibility to add motion-capture to characters. I tried to find tutorials on this but only found some for Maya so I used those and applied it on the "biped" in Max. I thought that maybe other people could use this as well and decided to turn it into a tutorial. It is my first one so hope it is understandable.
Before starting be sure that your character is facing the Z-axe which is very important for Motionbuilder.
When you finish rigging your model the first thing to do is to be sure your pivot points of the model and the biped are aligned. Then comes the boring part. To avoid dragging and dropping the different biped bones to the "character definition" tab in Motionbuilder you should try to adopt the naming convention that is being used in Motionbuilder for their characters (this isn't necessary though you can drag and drop the bones as well) for the naming convention these are the required names for the bones that have to be in a rig so that Motionbuilder can put a character to it:
If all goes well you should end up with a figure similar to this :