FBIK rigs must be exported from Maya with the current FBX plug-in version.
If you have the 2012 versions of MotionBuilder and Maya, you can use the single-step interoperability workflows to export FBIK rigs. See Send control rigs to Maya.
FBIK rigs exported from Maya with a version of FBX plug-in previous to version 2009.3 require the control rig settings Pull Iteration to be set to 10 to have full body mode.