These events unlock once you complete the 10 Media Op Challenges on each mountain. They will unlock in the order the mountains unlock so it doesn't matter which mountain you earn the challenges on. You will be notified which event is open and which ones you need to work towards on the mountain map.
Once you unlock one of these events you will need to beat the person at the top of the leaderboard. If the person at the top of the board has a score of roughly 20,000 you need to average higher than 20,000 per round. If your not confident enough use the practice option once the event screen to see how well you do for all three runs. Keep practicing as if you do not beat the high score you will have to try again once the leaderboard has been reset.