The B Dasher MK2 Is Not in Mario Kart Wii (It's in Mario Kart 8)

You won't find the B Dasher MK2 in Mario Kart Wii—it's a kart body introduced in Mario Kart 8 and Mario Kart 8 Deluxe on Wii U and Switch.

Mario Kart Wii has its own excellent roster of karts and bikes (like the Flame Runner, Mach Bike, and Wild Wing), but the B Dasher MK2 isn't among them. If you're set on driving that specific kart, you'll need to play Mario Kart 8 or Deluxe.