You have to use layers. Say for instance you import your video file, and your sound file.
You should then have two layers. What you need to do is create a new layer above the existing video(s).
Then right click on the empty layer and click "Generate Text Media" (or something like that).
Type out what you want, change the font size, style, color etc.
Then click the "placement" tab, and drag it to the bottom right/left or wherever you wish.
Then make sure to stretch the text layer from the beginning and to the end of your video... or wherever you wish the text to start & end.