This command loads the MotionBuilder presets provided with each release.
You can use this to load the MotionBuilder Import/Export presets without knowing their file path.
For example: FBXLoadMBExportPresetFile; loads "C:\maya2013\bin\plug-ins\FBX\Presets\2013.0\export\Autodesk MotionBuilder.fbxexportpreset" (The path may be different on your machine.)