Something that might help if you're interested, 'io sono bene', will not work as 'sto' literally means 'stay', in Italian, you ask people 'how do you stay?' And answer 'I stay....'. I think it's because they're asking more like, 'how are you generally feeling?' Learnt that on some lessons elsewhere. (:
With this program, it is always best to stick with the exact translations that have been previously taught. As in any language, for such a general question such as "How are you?" there are multitudes of correct answers, even just to express that you are well. In this example, Sto bene, apart from being an idiomatic expression, is actually very specific in its meaning. 'I'm okay.' is a slightly different idea, even in English, than 'I am fine.' As for the second answer you tried, I believe this is a simple case of adding the word 'very' which is definitely not found in the expression, 'Sto bene.' (otherwise it would be 'Sto molto bene.' I hope this helps and doesn't serve to further confuse you. Ciao!