After purchasing 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 O'clock position and point at the cave located there. If done correctly, you can select the map in the following step. Pause game play, then 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 you reach it the first time, it becomes unlocked at the main menu at the island rock.
Pause game play and save the game, then get into a competition game. If you lose that competition game, pause game play. Choose to load your previously saved game at the menu. You will not lose any money for losing that game.