Here is a tutorial for adding a gray box for text in your graphic using Photoshop Elements. I have Photoshop Elements 10, but the same procedure can most likely be used for other versions as well.
Please excuse the weird echo in the audio. I didn't know how to fix it and didn't want to do it over.
One thing I neglected to mention is about the layers. There are three layers in this file. From bottom to top, they are: the photo, the box, and the text. Whatever is on top is what will show up on top of the others. You can easily move the layers by clicking and dragging into their new spots if necessary.
I hope you find the tutorial useful. This is my first screenshot tutorial. I've watched many others and thought it would be fun to make my own. If you have any ideas for future tutorials, let me know!