Click on your vocabulary schedule on your dashboard -> Settings (gear icon) -> At the top is "Currently studying", click on the "Change these" -> this will bring up all the vectors for the schedule -> Select these vectors:
Note that Renshuu will not give you questions about kanji you do not know. As you study kanji in Renshuu, any words using that kanji will start feeding you kanji-related questions, so you'll gradually transition from studying the kana forms of words to their kanji forms over time.
The study vectors Anonymous123 is talking about will stay locked until you study some kanji, or otherwise mark the kanji in your vocabulary as known.
