Download the .kmz and drag-drop in 2D pane - you will find import information and much more in the FAQ the User's guide and of course, the 3D models page.
But the corner model, even if defined as a door/window during import, will require very exact positioning (in a thin wall) to work as expected. You may need to use "Service hatch" or something in addition to your model, if you want the cutout to work properly. Personally, I would prefer using two window-models joined at the corner.
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