When walking next to the bike, hold the lower button down. That should toggle the "walk" mode if it has one.
The cruise, if it's equipped and wired properly, and enabled, needs to be going in excess of 7 mph (I think?) and you hold that same lower button down over 3-4 seconds to engage. To disengage, hit any control (up, down, brakes). It will kick out by itself if it slows too much (wind, hill, etc.).