This question has been answered here before

When you are loading a model in SketchUp, you should always open SketchUp first, and then use the "Open OpenStudio Model" icon to load the OpenStudio model.

When saving, use the "Save OpenStudio Model As" option to save. You do not need to specify the file extension. I'm able to save a model using OpenStudio 2.1 plug-in on Windows 7.