If you insist on using an older version of SketchUp you need to learn some basic alternatives.
The Extension Manager updates etc and the Extension and 3d-Warehouse are not supported in older versions.
This means you can't install/update from the built-in links.
However, you can open the Extension Warehouse page in your web-browser and from there search for what you want and download an RBZ to your computer, and store it somewhere logical.
You can then use the Extension Manager's dialog to Install it [button near the bottom]...
This is not 'rocket-science'
A simple Google search shows that the Extension Warehouse is available here...
https://extensions.sketchup.com/