The button has a light to indicate that the system is on. That'll be your first clue. Once you're sure that's on, manually settle into the steady speed you want to go. Keep your foot on the pedal at that steady speed, and pull the stalk downward, holding just a moment while releasing the pedal, then release the stalk. If the system set correctly, it may dip in speed a moment followed by picking back up to your set point. If it does not set, be sure you are not also pulling the stalk toward yourself when pushing down - that's the Cancel position and would override also being pressed downward.
The system is electronic, and so it won't feel like the old vacuum based systems. If you just run up to a speed then yank the stalk while dumping the pedal, the system seems to be designed to think you didn't really mean to set it. I've noticed similarly that Cancel doesn't respond to an instant tap, but a momentary pull and hold. (Don't worry, the brake pedal DOES instantly release the speed.)