Download VirtualBox for your platform (Windows / Linux / Mac), and the Messaging Appliance. Alternatively, for cloud users, you can import this file into your cloud system, once it is unzipped.
Unzip the Appliance file, which should create a .vmdk file. (On a Mac, you will need to use the 'Unarchiver' app since it is a larger file than the native Mac Unzipper can handle).
Run VirtualBox on your platform, and add a 'New' project.