@peterwl777 said:
What am I missing here?
Do I need to buy the Pro version of Sketchup?
You are missing that SketchUp Free doesn't have the ability to use extensions. Nor do the other SketchUp for Web versions. You need a desktop client version to be able to use them. If you are just using it for hobby stuff you can still get SketchUp 2017 Make. If you are using SketchUp commercially you would need SketchUp Pro. Both are available at http://www.sketchup.com/download/all