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A .dmg file is a Disk Image, that needs to be mounted via double clicking on it. From that mounted Disk Image you can drag the actual application into the Applications folder.
Or do you have a problem doing that?
Maybe you could provide more information, like what Mac OS X version you use and what exact file you want to open and what you have done already?

Installing Applications in Mac OS X
 
A .dmg file is a Disk Image, that needs to be mounted via double clicking on it. From that mounted Disk Image you can drag the actual application into the Applications folder.
Or do you have a problem doing that?
Maybe you could provide more information, like what Mac OS X version you use and what exact file you want to open and what you have done already?

Installing Applications in Mac OS X

The file is Hitman Contracts, which I got from a friend.
The file is called : Hitman Contracts.app.dmg.
And I'm on Mac OS x Lion.

So when I try to double-click on it, nothing happens.. So how do I do then?

Thank you!
 
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