if i remember correctly the dungeon event is opened sometime after the meeting with the orca in the cabin. Either that or you need to observe the waterfalls (its the one near the bull forest) anytime after you go fishing with Snow.
holy crap... Im done fishing with Snow , met the Orca butler, and met the red lizard guy and still havent gone to the waterfalls.
Dang such tiny details can hold back some exp & money...
