Moved to Premiere Elements forum.]
Jon's suggestion will work in Premiere Elements also.
You can add this background right in Premiere Elements' Title Adjustments workspace by using the drawing tools and setting the color to semi-opaque black.
I see that the Title Adjustments drawing tool does not let you color with alpha or make a semi-opaque background.
To make a semi-opaque rectangle for your title background, you'll need to add the shape to a separate video track, between your video and your title, and set the opacity to 40-50%.
Placing the Text in Lower-thirds over a Shape is a great way to get what you want.
Another, that would not require a Shape (or a Color Matte), would be to apply a Style to your Text, that separates it better from the background.
The choices are basically asesthetic, and I use either often, depending the size of my font, my background, and other factors.