If you use SketchUp Pro, you certainly had to install .Net framework?
According to this guy, webdialogs work only if .Net framework is not installed:
http://robsworldoftech.blogspot.com/2010/12/getting-sketchup-8-working-in-wine-even.html
You can simply try it out by renaming your ~/.wine folder into ~/.wine_backup or something (for the case you need it again). Then Wine creates a clean new ~./wine folder when you install SketchUp 8 Free (+ the tricks for Internet Explorer etc, but no .Net framework).