1> Click gradient, then pick the gradient type that is circular.
2> Double click the color scheme for the gradient, choose your first color (doesn't matter what it is), set the opacity to 0%. Select your second color (whatever color you choose to have the picture fade into), and set the opacity to 100%.
3> Start in the center of the picture, then draw the gradient line to the outer edge of the picture...the second color you selected will slowly fill from 0%-100% as it makes its way to the outer edge.
I also apologize for instructions with no pics, I'm at work without Photoshop.
It's not that difficult, even for another PS n00b like me!
Mike