You can manually copy the plugin files to SketchUp's plugin folder, and then just restart SketchUp.

When you install OpenStudio (even if you don't have SketchUp installed) we place a copy of the necessary plugins within the OpenStudio installation. Within the OpenStudio install location go to "Ruby/Plugins". The files you need to copy are the "OpenStudio.rb" file and the "OpenStudio" directory.

Alex's post on this thread describes describe where within the SketchUp install you need to copy the files to.