Hi Frederick, if you want to set up a shortcut key for it, put in this for setting the CPlane:

script: moi.view.setCPlaneInteractive();

And this for resetting it:

script: moi.view.resetCPlane();

I originally had the dimensions go on a plane parallel to the cplane but through the first point that you pick. But that can be bad in other situations, see here for some previous discussion:
http://moi3d.com/forum/index.php?webtag=MOI&msg=9526.62

That also shows how you can modify the dimension command if you want to make it behave like I think you were originally expecting.

- Michael