To add narration to your video, click and drag on the narration section of the green timeline. This highlights a specific section in yellow for you to add narration. Once this portion has been highlighted, the "Add Narration" button (comic blurb icon) will populate for you to select.  (see example image below)

A record & stop button will appear, capture device dropdown, and volume sliders. Click the record button to begin recording and click stop when done. If you record longer than the length of the selection, the extra portion is not heard unless you make the narration bigger (by dragging the selection). (See image below)