User blog comment:PlantyThePottedPlant/Random Topic Blogging II: Electric Blogaloo/@comment-3006892-20120113211128/@comment-844113-20120113221452

It should be a button on your keyboard. You might have to press the Function (fn) button as well in order to get a screencap; I know it works that way on mine. It's usually in the upper right of the keyboard.