The Indigo for SketchUp (SkIndigo) extension is available in the standard .rbz format for SketchUp extensions. These steps will describe how to install the extension.
Download and install Indigo
Make sure you have Indigo Renderer or Indigo RT installed first.
Download Indigo for your system and install Indigo to the default location on your system.
You can download Indigo from here:
Download the SkIndigo .rbz file
If you haven't already, download the SkIndigo rbz file from the SkIndigo page
Install the .rbz File
On Mac, select SketchUp > Preferences.
On Windows, select Windows > Preferences.
Select the Extensions item on the left of the Preferences dialog.
Click on the Install Extension... button.
Select the SkIndigo-3.x.x.rbz file you just downloaded.
Click the Open button.
Click the Yes button in the 'Do you trust this Extension' dialog.
Check your installation
Once you have restarted SketchUp, SkIndigo should be installed and available to use.
You can check it by selecting Plugins > SkIndigo > Render Scene