As you play the challenge games and build up your zoo it will give you challenges, either photo or whatever the other kind is called, sometimes depending on what animals you have, like for the photo challenge where you have to take a picture of an ibex resting in a stable a jaguar swimming and a moose rubbing against the balsam tree(or whatever it's called... I have a horrible memory) I believe you atleast have to have one of those species, maybe two. For the non-photo challenges I've gotten one always at the beginning that says to raise $1500 in donations within thirty days. So all I can think of is to build up your zoo and wait.
Were you using the Musk Ox, Black Leopard, or Asian Elephant in your scenario? Because they don't count in scenarios, found that out with that "World's Biomes" scenario.