When an image is black and white, many of the editing tools will not work.
Therefore you have to trick the program into thinking that you are working with a color image.
With a b&w image in the Viewer mode, go to Tools/Modify/ Change Color Depth and select True Color.
You may have to save this new image with a new name, so as not to overwrite the original image.
Now you can add text, etc and then just save or save as the image when you are done .