Enter the 'Game Modes' screen and tap left on the d-pad 16 times. You'll hear a bell to signal that you've done that and now you have all of the venues. (This must be repeated each time you restart the game)
When chosing venue hold up on the d-pad until you hear a bell. When the match starts you will have both mini fighters.
From the Main Menu, Go into Create a Champ, select New Boxer. For the Boxer's First Name, type in the name GETFAB. Press Start. You can then cancel out. Fabulous will be unlocked for use in Play Now and Career Modes. Fabulous can be selected under the Heavyweight Division.