Go next to the tournament table in the Wild Corral casino. Use the map to find the northeast corner of the room and look at the floor. You will find a red poker chip that allows you to enter the Nebula. You may first need to play some of the Wild Corral tournaments. Enter the Air Hockey room from the right side entryway. Look along the floor of the right wall to find a red poker chip. Pick it up and you will be able to enter the Nebula.