After you beat the game, save the data when prompted (overwritting is OK). Once you choose to load that data, by pressing TRIANGLE will result in New Game+ where you can add in additional options for your new game to make it easier/harder along with new difficulties. Loading the save normally will put you back to the room before the final boss with a clown NPC. Talk to the clown and he will dissapear. Next go back to the place where you first met Rutee. The clown will be there. Talk to him and pick the first choice to enter the secret dungeon. Completing a floor will earn you EX points that can be used to purchase items from the clown (third option). You must choose to leave the floor when prompted, as dying inside will result in not gaining any points.