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I haven't added shaddow myself, but you could try this. Make a duplicate layer and then with the polygonal tool set to about a feather of 3 or 4, draw where you would like the shadow area to be. Now go to Image, Adjustments, Exposure and reduce the exposure slider a tad and then the Gamma Correction a tad until you get what you think is a realistic looking shadow. If the edges look to be a bit too sharp, repeat the process with more feather untill you get it perfect.
I'm sure someone will come along and show you a better way soon, but give it a try. I'm assuming you have a version of Photoshop CS of course.
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