You can convert .fbx files created in MotionBuilder2013 to an earlier version of .fbx file format by choosing from the main MotionBuilder menu Python Tools > FBX Export .

Python Tools menu
Saving MotionBuilder2013 to FBX version 2011 or FBX version 2010 ensures compatibility with Autodesk version 2011 or 2010 applications.

FBX Export