There is no "bum stomp" (ground pound) move in New Super Mario Bros. Wii. The game focuses on classic platforming: jumping, running, and using power-ups like the Super Mushroom, Fire Flower, Ice Flower, Penguin Suit, and Propeller Mushroom.
To defeat enemies or break blocks from above, simply jump on them. The Propeller Mushroom allows a mid-air spin that can stomp enemies, but it's not a traditional ground pound. Treat your console and discs with care to avoid damage.