Isn't the L C R pan control supposed to pan the instrument wherever you want it, just like an audio track? Doesn't seem to work with any Dimension Pro instrument in X3. Do I need to reinstall Sonar? Dimension?

There is no single pan control in DPro. Each element has a pan control, pan envelope and panners for FX. Testing the pan knob in an element is a simple process. Initialize DPro, drag a known stereo wav into element 1 and turn the pan knob. It should work the same as the pan control in the track.

Didn't understand the part about dragging a wave file into an element, but I can use the pan control that's there in Element 1 to pan the midi output. Thanks!
Where can I read up on what these Elements do and how to utilize them?

The Element 1 pan is actually panning the audio output of the first of four elements, MIDI processing is over by that time. There is a help file (chm format) shipped with the plug-in.  F1 may bring it up if the plug-in UI has focus. Otherwise the default location for the chm is either
C:\Program Files\Cakewalk\Dimension Pro\Documentation for 64bit installs
or
C:\Program Files (x86)\Cakewalk\Dimension Pro\Documentation for 32bit installs
 
If for some reason you cannot find the help they are available for download here http://www.cakewalk.com/Support/Knowledge-Base/2007013200/Dimension-Pro-1-5-Help-files