Viper is available as both command line tool and IDE. The easiest way to get Viper is via the pre-compiled package.
The Viper IDE is based on Microsoft Visual Studio Code. After installing VS Code, open the extensions browser (⇧^X or ⇧⌘X). Search for "Viper". Install the extension, and agree to restart VS Code.
Download and unzip the pre-compiled package for your platform (see table below). The Viper IDE will be activated once a .vpr file is opened in VS Code.