To add a subtitle, place the timeline cursor at the position you want the subtitle to start. Then you can click on the "Add" button. You can also set the default duration of the subtitles added changing the default value of this button.
To remove a subtitle, select a subtitle clicking on the subtitle you want to select, and then click on the "Remove" button.
The following is a quick video showing where to find the "Add" and "Remove" button.