Just drop the icon and the scriptlet in the BZ_Dir folder and restart Sketchup.

Note that the 'Precision' parameter is a number of segments for a full circle (not the arc), which usually is 24 (default value). I just performed a quick correction to avoid having too many or too less intermediate points, based on the pixel precision on screen (so if you zoom, you get more arc points). As a standard function of BezierSpline, you can of course edit the curve or convert a Polyline into a Courbette (via the contextual menu).