The best way to search for reference material is first to decide which book is the most appropriate for your particular need. For example, if you want to know another word for "happy," you can go to Roget's II: The New Thesaurus and type that word into the search box. If you're looking for which Shakespeare passages talk about a rose, go to the Oxford Shakespeare page and type "rose" into the search box.
Another way to search for something is to type in a keyword on the front Reference page and see which reference book provides results.
If you are unable to find specific information, first check and make sure that your spelling is correct. If your spelling is correct and you are still unable to find what you're looking for in one of the Yahoo! Kids Reference books, try searching the Yahoo! Kids directory.