I know it's confusing, but you must install the OpenStudio Application, not the OpenStudio SDK.

Re-read the wiki at: https://github.com/openstudiocoalition/openstudio-sketchup-plugin/wiki/OpenStudio-SketchUp-Plug-in-Wiki

Get the 1.6.0 from https://github.com/openstudiocoalition/OpenStudioApplication/releases/tag/v1.6.0.

The OpenStudio Application 1.6.1 will also include internally the OpenStudio SDK 3.6.1, so no need to install it separately. The OpenStudio Application has - in addition to the Graphical User Inferface - special ruby bindings that the OpenStudio SDK does not have, and that the openstudio-sketchup-plugin needs to work.

Point the OpenStudio Root Directory to C:\openstudioapplication-1.6.0 once you have installed the OpenStudio Application 1.6.0.