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".