"When are you free?"
As Helgemonster pointed out, itsu means "when", but tada sometimes means "free", just a different meaning of "free".
In this exercise, we are learning ひま which means "free" as in "having spare time". The hints (which I agree are terribly unhelpful) ただ and 無料の (muryou no) mean "free" as in "no cost". Also, the other hint that I get, 自由な (jiyuu na) means "free", but in the sense of "unrestricted" or "liberated".
It's also worth noting that ただ also means other things like "just" and "only" in other contexts. In my experience though, when you receive something for free from a store or restaurant, サービス (saabisu) is more commonly used than ただ, possibly to avoid ambiguity from these extra meanings.
I struggle with n desu as well. Right now I think it may be used with other question words to get the reason for something. In this same lesson the word なぜwas used and んです was used there. For example, why is it that you were late? This probably makes no sense as I am trying to figure it out too. Maybe a Japanese native or expert can help us. :)
In another sentence, hima meant "bored" or "boring". I guess this could be interpreted as "When are you bored?" since as you are bored you probably don't have anything else to do.
Would「いつが暇ですか」work? I feel like が after a question word works most of the time.
That would make "when" the subject, which doesn't make sense. ひまがいつですか is actually what I put but was marked wrong, and to be fair I can't find any evidence it's a normal way of saying it.