I had a look at the various options a long time ago, and the two I would suggest would be Chessmaster and Fritz DS. Both of these allow you to set the strength of the opponent and don't distract you with "fun" graphics jumping all over the screen. If you don't have the means to test them out and just want a purchase recommendation, I found Chessmaster to be a more complete package.
However, both of these share the problem that is present in all chess programs, which is that at a lower strength level they play in a very un-human-like manner. They'll occasionally make really good moves and occasionally make terrible ones, only the scale is tipped towards the latter as you set the rating lower and lower. As a result, if you are planning to use this as a way of learning the game to play against other humans be away that a real person will play very differently.