Instructions
Head for the southwestern section of Veilstone. Enter the house directly to the left of the one containing the Massage Expert.
Speak to the clown and play his guessing game. Keep playing until you guess correctly (it shouldn't take more than a few tries) and he will give you a coin case.
Go to the game corner, which is found near the center of Veilstone. Play the slot machines to accumulate coins. Unlike those in the real world, you should be able to win at these to increase your coin collection.
Purchase Hidden Power once you have earned 6,000 coins.