To install the Wisext tools you have 3 options:
1. SketchUp Extension Warehouse
The Wisext tools are now available on the official SketchUp Extension Warehouse:
You access to that library directly from SketchUp by clicking this button:
or 
You don't need to install each tool. You install one and you can activate the others by selecting in the menu:
Extensions > Wisext > Activate extra tools
We advise updating the plugin to get the most recent version of all the tools:
Extensions > Wisext > About update
2. Installing a .rbz file
SketchUp installs the plugin if it is zipped in a .rbz file. You can get it here.
From SketchUp 2017, open the Extension Manager and click "Install".
or 
3. Windows installer
You download a .exe file installing the plugin. It works only for windows.
You get it here.