OpenDD is a Java multi-platform software, available for most flavors of Windows (including Windows 7 or XP) and Unix (including Linux and MacOS). Please check that Java is installed on your system, and download the OpenDD installer with the link below.
Save "installer.jar" in some place easy to reach (e.g. your desktop) and run it by double clicking on it. Then follow instructions.
To start OpenDD on a Windows machine, simply double-click the OpenDD shortcut that was installed on the desktop. You can also drag and drop a previously saved campaign file on the OpenDD icon to directly open the campaign.
To start OpenDD on Linux (or any Unix based computer), just run the "opendd" command, located in your installation directory. Of course you can copy this command to some convenient place on your PATH, or use a symbolic link.
To start OpenDD on Mac, just run the "opendd" command, located in your installation directory.