Rocket Slime help

im the only one of my freinds who has this game,
so we got our DS' out and tried to play multiplayer, but they downloaded the game and started to play by themselves...

which gets me wondering... can i play with my friends with one card?

No, they cannot. You guys need your own cart to play the tank battles.

However, you can send a demo of the tank battles to your friends' DS.