Hi Playdo - when surfaces are joined you can't turn on their control points, that's because the way NURBS surfaces work the joined pieces may only be sharing a "trim edge" where they touch and not have their underlying surfaces control points actually touch.

There's some explanation of this here:
http://moi3d.com/faq#Q:_Why_does_show_points_work_for_some_objects_but_not_others.3F

Because of the way surfaces work by having "underlying surfaces" and "trim edges", it is usually not part of the workflow to edit solids by pulling control points around, with NURBS modeling you usually only edit curves that way and not solids. Usually if you want to edit a solid you delete it and edit some of the curves used to construct it and then re-construct it (by sweeping or whatever you were using) to make the updated one.

The overall workflow is far different from how polygon modeling works - it's much more of a focus in polygon modeling to tweak control points on 3D objects, and if that's really the style that you want to do (which works well for organic models), then you would want to be using a polygon mesh style modeling program to do that kind of work and not MoI. MoI is stronger in much different workflows than that - in MoI it's more of a focus to focus your drawing and editing effort on a few curves, then construct objects from those curves and slice things with boolean operations. The whole "underlying surface" and "trim curve" type methods that get in the way of editing control points of solid is also what makes boolean operations work so much better in MoI than they do in a polygon modeling program - in a poly modeling program booleans are very problematic.

- Michael