The easiest way to get Viper is via the Viper IDE.
The Viper IDE is based on Microsoft Visual Studio Code. After installing VS Code, open the extensions browser (⇧+Ctrl+X or ⇧+⌘+X). Search for "Viper". Install the extension, and agree to restart VS Code.
The IDE will be activated once a .vpr file is opened in VS Code. The dependencies (~100 MB) will be automatically downloaded and installed in your system. Note that the installation requires administrative privileges.
* Mono JIT compiler is only required for the Carbon verification backend.