It's easy: either de-select any currently selected entry, or click one of the top-level entries, then click New Sibling, and a top-level entry will be created. De-select an entry by Cmd-clicking on the selected one, or by clicking in an empty area of the list.

If there is an entry selected, the New Child button will create a new entry within it, inheriting its icon and custom fields - a very handy feature.

You can also drag any entry to any level: make it a top-level one, or within an existing one. The drag destination indicator is a black box when dragging onto an entry (to drop it within), or a black line with a circle on the left to drag to that location. The circle indicates the level to drag it to; move the mouse to the left and right to choose the level if ambiguous.