Open up a photo that need shadow and highlight adjustments.
Go to Image » Adjustments » Shadow/Highlight.
When the Shadow/Highlight dialog appears, the default shadows amount is 50%. You can increase the amount if you want it lighter, or decrease if you want it darker.
If you want greater control over the shadows and highlight of the image, you can check the "Show More Options" box at the bottom. It will give you an array of sliders for adjusting your image.