If your exposure is over or under exposed you can make adjustments with the exposure slider in Camera Raw. I had already worked with this image and saved it as a tif. In Photoshop if you go to File – Open As  the image can be opened in Camera Raw like the original raw image.

File-Open As

To select a tif or jpg file by left clicking on it.

Select the tif or jpg file to open in Camera Raw

In the Open As dialog box to the right of Open As at the bottom of the dialog box click on the arrow and select Camera Raw for the file to open in Camera Raw.

Select-Open As-Camera Raw

I had already processed this image. In order to demonstrate how to make an exposure composition I have moved the exposure slider the left to -0.50 which  under exposes the image and makes the image darker. If I had moved the exposure slider to the right to +0.50 it would make the image lighter and over exposed the image.  

-0.50 exposure Slider

I moved the exposure to the left +0.50 stop to 0 to lighten the image to the exposure I liked best. This is also the exposure that I captured in the Camera. I try to expose and crop the image in the Camera to create the best possible file. My goal as a photographer is to use Photoshop to make a good image better not make a poor image usable. In Photoshop Elements 11 and CS6 you can make adjustments up to 5 stops  plus or minus but I find that if you have over of under exposed the image by more than 2 stops your recovery efforts will brings poor results.  

xposure slider adjustment to 0.00