Its hard to construct curved line in sketch-up accurately.This plugins might helps.
BezierSpline draws a variety of Polylines, Bezier and Spline curves, all in 3D. The present release gathers all extensions, including those published recently (Courbette, Catmull, F-Spline).
download it and paste it to your plugins folder in sketchup for example like this:
C:\Program Files (x86)\Google\Google SketchUp 6\Plugins