Trex
OK I don't know if you can do this in Elements 6 but here is a technique that works in Photoshop.
Make a new layer, and select "Soft Light" as layer mode
Now select the brush, and set the opacity yo about 15% and the flow to a bout 25%.
Select a brusg size that's about right for want you are about to do.
Make sure your foreground colour is set to black, and start painting over the lighter areas. All you need worry about is not going over the different parts of the picture. By setting the opacity to 15% the maximum density you can lay down is 15% no mater how often you go over the image, provided you hold the mouse button down. OK 15% isn't a lot but release the mouse button, click and paint again, another 15%. You can choose less if you want. The Flow rate also helps when it comes to those areas that are dark enough all ready