ART is based around Maya and not MotionBuilder. The ART rig is not directly compatible or exportable to MotionBuilder.
For Setting up a Rig in MoBu, you’ll want to import a clean FBX with just your character’s skeleton and mesh (basically what you’d import into the engine). You’ll need to then characterize in MotionBuilder. More info on that here: Characterization in MotionBuilder -3dmotive - YouTube
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