Super Monkey Ball controls a little differently to most other games: instead of using buttons to move your character around, tilting your device will move your character in the appropriate direction.
There is also an option to show a Tilt Meter during the game. This is a crosshair with a red dot in at the lower left of the screen, which shows some extra feedback on your controls. To turn the Tilt Meter on or off, first select "Options" from the main menu, then chose the "Tilt Meter" option. At the bottom of the screen chose "On" or "Off", as appropriate.