it dedends on the level, you usually have to not wipeout at ALL and make it in a certain amount of time. it will also tell you exactly what time you need and how many wipeouts on the challenge mode...
You can't, sorry. It would probably require a whole new game, or at least a download. Besides, treyarch stopped it because they doubted it would work :'(
Unfortunately for Wii owners, there is no zombie mode. Call of Duty's World at War Zombie Mode is specifically for the PlayStation 3 and XBox 360. Obviously considering the Wii is more directed...