To enable Chance text, right-click inside a track and enable "Chance text".
Chance text format: xx, where 0 = Only play if the previous note played. For instance if you want to play a chord with same chance. 1 = 0.004% chance. .. 40 = 25% chance. .. 80 = 50% chance. .. c0 = 75% chance. .. ff = 99.6% chance. = 100% chance. (empty)