When you install OpenStudio v1.6.0, the SketchUp plugin is installed to C:\ProgramData\SketchUp\SketchUp 2015\SketchUp\Plugins. However, if there is an OpenStudio plugin in your AppData directory, it will override the ProgramData location. Check that there's no plugin in %APPDATA%\SketchUp\SketchUp 2015\SketchUp\Plugins.