Unfortunately, there’s not an intuitive "Add new track" button or option in ScreenFlow.
Instead, you need to drag the always-empty bottom track to the top. This will give you a new track on the top, and it will automatically generate a new empty track on the bottom (which can be used to create new tracks in the future).
To drag tracks, click the three little bars icon.
To delete existing tracks, right click the three little bars icon and choose "Delete Track" (this option is available for any track except the bottom one).