On the left-hand side of the screen there is a tree view of all the library items that can be used in the template creation: The General and Tables Group items apply (and activated) only when forming the Header and Footer area, whereas the Depth Related Group Items are activated and can be used on the Depth Related Area pane.
Library items are only activated if a design panel is selected which can use them. In the image above the depth related area is selected and so relevant items are available.
Groups can be collapsed or expanded by clicking on the triangle next to the folder icon:
Note that some groups are only activate when working within an appropriate template type (e.g., Site Plans).