Matthew,
Another is to make sure you are installing MathWorks' Robotics System Toolbox, not Peter Corke's Robotics Toolbox. rigidBodyTree is only in the Robotics System Toolbox.
Finally, you can try running these commands to reset the MATLAB Search Path to the default value:
restoredefaultpath
rehash toolboxcache
If that fixes the issue, you can save this for future sessions with:
savepath
-Cam
