The easiest way is to create a new item by simply typing into an empty placeholder - simply select it and start typing an item caption. New placeholders will be automatically created, so you can insert next items easily.

If you want to create a new element in an arbitrary position, you will have to create a new placeholder for it. In order to to that, select the desired position either by clicking it with a mouse or by using arrow keys on the keyboard and insert a new placeholder by pressing the Insert button on the toolbar or Insert key on the keyboard. You can also right-click in the desired position and choose Insert from the context menu to achieve the same result.