The default installation directory for the legacy plugin is C:\Program
Files (x86)\Google\Google SketchUp 8\Plugins. If you want to use the
legacy plugin with 2013 try installing it to C:\Program Files
(x86)\SketchUp\SketchUp 2013\Plugins instead. We could change the
default installation directory but then people with SketchUp 8 would
have this problem.....
Hope that helps,
Dan