Often these will include the application itself, some form of arrow, and a shortcut to the Applications folder.Double-click the DMG file to open it, and you’ll see a Finder window.dmg installation file from the official link on above Like if you wanna download Outlook Express for mac from this page, you’ll directly get the. Most Mac OS applications downloaded from outside the App Store come inside a DMG file. Only works on Mac OS 8.1 or 9.x systems.Advertisement About Outlook Express App for MAC