Once that is done you have to add physiques (or skin) to your model. Arrange the envelopes etc like a normal rig. Once you finish that select the biped and the model and go to "export" (File -> Export) select to export it to .fbx (Motionbuilder file).
You get this screen:
If you only want to export the model and the bones you chose "geometries", "geometries used as bones, exported as bones" and "skins" and click ok. You can also export everything J that is up to you but lets focalise on the characters.
Once that is done you can close Max save your Max file and open Motionbuilder.
In Motionbuilder open the file you made before "yourfile.fbx" and wait for Motionbuilder to load. if you have the rig standing on the ground and the model itself half way through the ground recheck your pivot point in Max (the file will work but it will be a real pain to animate).
When everything is loaded go into the asset browser and chose the "character". Drag and drop it onto the model. Normally you get a pop-up in the screen with the question: "characterize"
Click on it. Normally if you followed the naming convention the character gets made right away and the character is being characterized and you can animate it right away because a control rig has been created automatically. If not you get an error screen with the missing bones that you will have to place by yourself (see further down).
If you didn't follow that naming convention or if you have bones that didn't take the right place or so go into the navigator and go down to the "characters" drop down. double click on it and you see two "tabs" Character Definition and Character settings, the check box Characterize is unchecked (because of the errors) or checked if the characterisation went well. (If you want to change something to the rig like add some bones for the fingers or so uncheck it otherwise you can't modify the character).
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