Renshuu will not test you on kanji that you don't know. In order for your vocabulary quizzes to start intergrating kanji, you need to either make a kanji schedule and start studying kanji, or use the Kanji Index in the menu to mark any kanji that you knew before starting Renshuu. Once kanji are flagged as known, they will replace the hiragana in your vocabulary.
It is NOT recommended to enable "Show kanji for words commonly written in hiragana" (which will force you to study the rare kanji forms of words like これ and する) or to set "Show unknown kanji" to the option without furigana (which will disable furigana globally whether you've studied the words/kanji or not, and is severely overkill if you don't already know 1000+ kanji).
