To unlock it, you must beat story mode. After you unlock the mini-game you will find the game playable in the Back Room with Whomp. The mini-game is a 9 square layout which is separated. The four starting characters pick one of the number square, 1-9. After the four character pick them. The unused characters and Bowser pick a square as well. Now a dice block will appear over all nine characters. Here comes the fun part, after everyone has hit a dice block, the numbers correspond with the square of the same number will collapse. The characters on top of the square(s) will gasp, and from a top view, will show them fall and scream. The players continue to hit the dice block until all other 8 character fall.
There is also a very good chance that there will be no winner. Lets say Mario is on square one, Yoshi on square five, and Wario on square eight. Mario hits an eight, Yoshi hits a one, and Wario hits a five. Thus, no winner is determined.