The search engine is located on the right side of the toolbar. If you can select the search box and type a word inside that, it means your search engine is ready to use. Otherwise, Kiwix will ask you if you want to index the ZIM file or not. You will need to confirm this process in order to use the [[Special:MyLanguage/Features#Search_engine|full-text search engine feature]] or cancel it and use the program without this feature.