This is so simple!  Change your top Styles swatch to a solid color.  I used white.  Add a new layer and select an area that covers where the labels are.  Flood fill with white.  Move the layer to the bottom.

You are ready to add text to the labels.  Click on the text tool and make the changes you see below.  You want Floating/Antialias text.  Click on the Standard Text button and choose your text color in the Fill swatch.  Don't forget to choose the font you want!  I used Arial/Bold, size 10.  Now if you aren't sure if you will leave the text in one spot, while the text is still active, cut and paste as a new layer.  Then you can do whatever you want with it later.