The Hierarchy tool window is not shown when there are no hierarchies to display. You have to build hierarchies first.
Refer to the section Building Hierarchies to learn how to build hierarchies.
- On the main menu, choose .
- Use Alt+8Command 8 keyboard shortcut.
- Right-click the currently displayed tab, and choose / on the context menu.
- Use the Alt+RightCommand Right and Alt+LeftCommand Left keyboard shortcuts.
- Click the currently displayed tab, and choose the next tab to display.