Under "Presets," click on the third box from the left on the top row; this is the "black/white" gradient. Click OK. Then click and drag through the transition area of the image - where you want to put the "graduated" part of the filter - starting from the top. You may have to do this several times to get it right. You can then double-click on the Curves adjustment layer icon in the Layers Palette (or the Levels icon in Photoshop Elements) to further adjust the curve.
You can use a similar technique with two originals (one exposed for the highlights, one exposed for the shadows) or two RAW conversions of the same file. With both images open, use the Move tool, hold down the shift key and drag one image on top of the other (using the shift key makes the two images register precisely on top of one another, as long as both images have the same pixel dimensions). You now have two layers: one a darker version of the image, one a lighter version. Add a layer mask to the top layer (Layer > Add Layer Mask). Select the gradient tool, choose the Black, White linear gradient as explained above, then just click and drag where you want the transition between your two images to be.
Article by Michael Frye. To learn more about photography, explore the many online photography and Photoshop classes offered here at BetterPhoto.com.