@Lunarmoon21 Hi! I moved this thread to the Other Pokémon Games subforum. As SAF pointed out, it fits better here. Please be sure to double-check the intended purpose of each subforum! It keeps the place cleaner and allows people who can help you to find your thread more easily.
As for the problem at hand, Bulbapedia says you'll need to follow these steps to unlock the dungeon.
1. Finish Mt. Freeze
2. Complete three missions
3. Talk to Wynaut and Wobbuffet outside the Pelipper Post Office
4. Accept the "Punish Bad Mankey" mission. That should unlock the dungeon, since you'll need to go to that dungeon to do the mission.