I've crerated masks before in Motion without any problems. However tonight I drew after creating a mask (with footage sent from FCP), I couldn't access any control points or even the mask iteslf. This happened after deleting the mask and trying again, and even after quiting and then relaunching Motion. Another problem is that when I click to add a control point, the point is more than a few pixels away from where I clicked.
While we're at it, I seem to get more than a few force quits in Motion - could this be our video card, an ATI 9200 (I think).
When you say that you can't access the points on the mask, do you mean that you can't see the points, or that they're there, but you can't interact with them? The two typical methods for going into the points-editing mode is to either double-click on the mask in the Canvas, or right-click on the mask (again in the Canvas) and choose 'Edit Points' from the menu. Pressing Tab repeatedly will cycle through the editing modes, eventually arriving at the 'Edit Points' mode, as well.