As Slider257 said, the mini game is in Subway Town. You'll find a man with a guitar sitting at the top of one of the staircases, and when you look at him a dice icon will appear.
The game is entirely random, and based on memory. It will show a board with the four buttons A, X, B, and Y. He will play notes and those buttons will flash. All you have to do is play the same buttons as him past 5 rounds and you get the achievement.
Now, I don't have a very good memory. I failed it on round 3 consecutively. The good news is the patterns continue from the last round. Slider257 suggested writing down the patterns, but later in the rounds they get faster and faster. When I played it, I just recorded the patterns with my iPod and played them back to write them down. While recording, I managed to get the achievement first try. Good luck!