After Murray wins the race at the dog track, Sly enters the Boneyard Casino. The casino is flooded with water and trashed up, like the rest of Mesa City. When Sly finds his way to the end of the level, there is a giant spinning roulette wheel. He needs to keep jumping along on it and hitting the green key spot. After hitting it three times, the glass dome in the center breaks and Sly can retrieve the key. After that, he moves on to the next level.
Once you've completed the Master Thief Sprint challenge for this level,you will be able to hear a commentary of members of Sucker Punch Productions stating that Sunset Snake Eyes was originally supposed to be all casino but was later changed.