To install the Wisext tools you have 2 options.

1. SketchUp Extension Warehouse

The Wisext tools are available on the official SketchUp Extension Warehouse:

You access to that library directly from SketchUp by clicking this button:


2. Installing a .rbz file

SketchUp installs the plugin if it is zipped in a .rbz file. You can get it here: newWisextRelease.rbz

Open the Extension Manager and click "Install".