I read a ton of incorrect posts while going insane trying to figure out how to start a new game if previous data exists. Here is the correct way:
Boot up the DS, touch the Dogz Pack icon to load the game. After the Ubisoft logo fades it the game will be on the green title screen. Now, hold down at the same time L+R+X+B+Select You will then be asked to confirm your decision, you have to touch "O" to confirm (not the other option "x")
Hope this helps, most incorrect posts were forgetting the select button. Its a finger twister, but it works.