Also some more information on setting the cplane here:
http://moi3d.com/2.0/docs/moi_command_reference5.htm#cplane

The little axis widget placement tool is used in a few different commands like Orient as well, some info on how that picker works here:
http://moi3d.com/2.0/docs/moi_command_reference11.htm#orientationpicker

If you want to orient it to have z perpendicular to the plane of 3 points, you use that method above though - place the orientation picker at one of the points, drag the x axis of the picker to point at one of them, then drag the y axis to spin it around the x axis line and align it to the other point.

One last note - the various planar drawing commands including the arc drawing command also will try to use an object that was snapped on to with the center point as a temporary construction plane to orient the arc when you are using the 3D viewport to place the points. This is to allow some easier direct drawing on a surfaces without necessarily needing to set a construction plane. But in this case that will probably get in your way which is why you want to hide the sphere so that you won't get that happening.

- Michael