Actually, in SketchUp you can't change the default number of segments for circles or arcs. And you need to make the change to the segment count prior to drawing the circle (you have up until just after you click for the center of the circle but before dragging out the radius or before clicking to set the first point for an arc) or arc or, as Tim notes, you can change it afterward in the Entity Info box.
If you make the change prior to drawing the arc or circle, that change will remain in effect for that drawing session. When you next start SketchUp, however, it will revert to 12/24 segments.
Did you look at the link I gave you yesterday? Did you download the plugin suggested there?
Last edited by Dave Richards; 01-12-2010 at 11:11 AM.
Irony: The opposite of Wrinkly