To render with Indigo you'll need to install the actual Indigo application, and an exporter for your preferred 3D modelling application.
The Indigo installer comes bundled with installers for the latest SketchUp, Blender, Cinema 4D and 3ds Max exporters. On running the Indigo installer, it will attempt to auto-detect which of these packages you have installed and launch the installers for the corresponding exporter modules.
The latest stable release of Indigo and the exporter modules can be downloaded from http://www.indigorenderer.com/download.
We also regularly provide beta versions, generally with good stability, on our News and Announcements forum. These are recommended for confident users, or if you experience any problems in an older release.