They only launch during daylight hours between 9-3 or something like that. Also you won't see them if the graphics are turned down. I don't know which setting is required to be turned up but they launch fine for me around midday in game time.
My graphics are full, and I know about time but as I see they only flying when residents are at park but residents are almost always at 0 and there are a lot of kids. Every few days I got 3-4 baloons.
I have the DLC for hot air balloon park but I cannot find the simcity lunch park!!! is this a bug or something EA intended for those who purchase DLC to not have?