http://www.ofzenandcomputing.com/how-to-install-dmg-files-mac/

Software applications are usually distributed (downloaded) as .dmg (disk image) files.

You need to 'mount' the .dmg file, usually by just double-clicking on it.

Installing is usually just copying from the mounted .dmg file to the Applications folder (or a shortcut to it).

1. Find the downloaded file, which usually ends up in your Desktop or Downloads folder
2. Double-click the .DMG file to mount it. A new Finder window showing its contents should appear
   1. If the window also contains a shortcut icon to Applications, drag and drop the app onto the shortcut
   2. If not, double-click the mounted volume on your desktop and drag the app icon from there to the Applications icon in the Finder sidebar