Additives help you maintain microbial activity within the pile, speed up decomposition, balance out pH levels, prevent fly strike, negate damaging effects of pharmaceuticals on the compost, and generally prevent problems from arising.
Composting toilets are designed to keep flies away, and with proper maintenance flies shouldn't be a problem. If, however, you've found flies buzzing around your toilet, you need to deal with them sooner rather than later.
A well-designed and properly maintained toilet shouldn't have any unpleasant odours. If it does, something is wrong. Here are a few reasons as to why a composting toilet may start to generate odour, and ways to fix it.