After buying the Hidden Cave mini-game at the store, go to the practice table. Start a game, and point towards where the hidden cave is supposed to appear. From the break position, go to the right at about the 2 Oclock position and point at the cave located there. If done correctly, you will be able to select the map in the following step. Pause the game and select the "Inventory" option followed by "Sub Games". Then, select the Hidden Cave map. Answer "Yes" when prompted to quit the practice game. You should now be at the Hidden Cave, with a treasure chest with money. After reaching it the first time, it becomes unlocked at the main menu at the island rock.