So you must use Plugins of Fredo Joint PushPull, Curviloft
Shape bender by Chris Fullmer...
http://www.youtube.com/watch?v=tGHTIOMm_34
http://www.youtube.com/watch?v=holafltwWrc
http://www.youtube.com/watch?v=qsKZpmrJkxs
or this one (there a free version!
Minute 3.50
http://www.youtube.com/watch?v=UviJRlu_00w
Or use intensively the SketchUp native tool: Sand Box
