To the righthand side, you'll find a list of tools which includes the Vignette slider (and associated tools). To add a Vignette, simply just drag the slider left or right (right will give you a dark ...
The recently released Photoshop Elements 12 isn’t hard to use. However, if you want to learn all its bells and whistles, you’ll want “Photoshop Elements 12: The Missing Manual” from O’Reilly Media.