Nautilus 3.5.4 and later
William Jon McCann removed this feature in version 3.5.4:
Use a list model instead of a tree model
It is the list view after all. Tree models don't work well on touch and it isn't consistent with the file chooser.
Nautilus 3.4.2
Select menu View then menu item Sidebar and then select Tree.
Toggle the sidebar between hidden/shown with F9
