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:
or
If you have already installed a tool (Find And Report or 3D Offset), you can access the others by going to the tool menu in the Extensions menu and clicking on "Enable or disable tools".
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".
or