Yeah... if they guess wrong they don't get the points. It's kind of random, but Korean students are well versed in the concept of "prediction". I usually tell them to use their X-Men powers to guess; that gets them all worked up. With other bomb games if the group in question guesses incorrectly, I give other volunteers a chance, but obviously that won't work here, as there are only one of 2 options. But shake it up in terms of the 'icons', i.e. the various mario characters. For example, let the grenade bomb mean they loose all their points, the bullet - another group loose their points, the Venus fly trap - steal 2 points from each group, the ghost - switch points with a group, the brown mushroom manthingy - switch 2 other groups' points, the beast - steal 5 points from any group, etc.
Oh, and let them play it twice, they'll beg you for it. Then it becomes more of a memory game and they really nail the target language, and it will take up 30 minutes as opposed to just 15.
Hope this helps,
A