I haven't seen the film, so this is from the book.
The bees (called Tracker Jackers) had been subdued by the smoke from the earlier fire, so weren't particularly active or noisy. Katniss herself hadn't noticed them because of this fact, but has her attention drawn to it by Rue from a neighbouring tree.
It's only in the morning, when the effects of the smoke have worn off, that they start to become more active and aggressive, and this is when Katniss uses it as a weapon.
I would guess that they're simply one of the many dangers present in the arena, and Katniss was fortunate enough to find herself stuck in a tree with a nest full of subdued bees. Given that the Gamemakers really have no interest in a particular tribute winning, I can't see why they would have helped her out of that situation.