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Did you buy the Retail version of Snow Leopard ? Like this one --> http://www.apple.com/shop/product/MC573Z/A/mac-os-x-106-snow-leopard
The Retail Snow Leopard has white color and has a picture of Snow Leopard on it.
There are some non-retail discs also know as restore DVDs. The non-retail disc will not work on your Mac (as they are for some specific macs) and the non-retail disc has gray color. If you bought this disc, then you should return it. It will not work on your Mac.
If you bought the Retail disc, then put it on your Mac. Restart your Mac and immediately hold down C key on your keyboard for 20 seconds and this will boot into installation screen. Then just follow the onscreen instructions to install Snow Leopard.
If you have any questions or would like any additional information, please don't hesitate to ask