Right-click a bone in a CATRig. > Tools2 quadrant > FileIO > Import HTR/Import BVH
Select CATParent. > Motion panel > Clip Manager rollout > Clip mode > Click Load button. > Set Files of Type to MotionAnalysis HTR(*.htr) or BioVision BVH(*.bvh).
You can import HTR and BVH motion-capture files into 3ds Max CAT using the same method. The respective importer opens the file as a hierarchy of Box primitives with keyframes. You use the Capture Animation tool to map this motion onto your CATRig.