To search the selected node (and its child nodes), the entire tree, or the entire document:
(1) First, click "Menu: Search/Tree",
(2) The "Search Tree dialog" appears. Select the options you want; and type in the text that you would like to find.
(3) Press Enter, or click Find.
(4) The search results appear as a list of items in the grid. Each line corresponds to an article/node. To navigate to the article containing a result, just double-click on the line corresponding to that article/node.
(5) To close the "Search Tree dialog", you can click on the "Close" button, or press the "Esc" key on your keyboard.
Alternative ways of initiating a tree search
Instead of clicking "Menu: Search/Tree" as described above in point (1), you can also use the "Search tree" toolbutton, or press "Shift F11" on your keyboard.
The keyboard shortcut "Ctrl F" will also activate the "Search tree dialog" but only if the tree has focus. If the article has focus, then "Ctrl F" will open the "Search article dialog" instead.