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 O'clock position and point at the
cave located there. If done correctly, you will be able to 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 reaching
it the first time, it becomes unlocked at the main menu at the
island rock.
Trainer: +1 by Deviance