Where do I play "The Shell Game"?

An even better trick is to relax (you don't even have to look at the screen), let him do his thing and then when it's time to pick, pause the game, the x-ray vision will let you see which cup the ball is in!

UPDATE: These locations are actually Unlocked when you free gain access to the ctOS towers. They are parts of the points of interest that will then become available within the area showing up as the dice icons as mentioned before along with other points of interest in the vicinity.

As you roam around the city you will eventually come across people all over the place who want to play this game. They are denoted on the minimap (and likely map) with the symbol of Dice on them.

After that you are on your own in how to play, its simple. Watch them put the ball under a cup and the try to keep track of it as best you can to pick the right one. The game has ranks that keep getting more and more expensive as you play.

UPDATE: As pointed out by @Philipp in the comments, you can still use your focus to slow time during this game to help you track the movement better.