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Do you mean matte vs. glossy? You only get a choice with the MacBook Pro, the black/white MacBook is always glossy.

For the MBP, it will be in the list of options that shows up as soon as you click on a base model.
 
The macbook can only have a gossy screen, but if you mean the Macbook pro, you choose the screen after you select which Macbook Pro you want to buy.

Edit: iMeowbot beat me to it.
 
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