In Mortal Kombat (aka Mortal Kombat 9, 2011 version) for the PS3, and Xbox 360, you may have noticed that there is no on screen prompts to select an alternate costume when choosing your character.
First, after any costume is unlocked or purchased, you have to visit the Nekropolis in the Extras menu and click on that particular character to fully unlock the costume.
Once that is done, in order to choose an alternate costume, when you’re on the character select screen you just need to press the Start button when you have your character highlighted, and an options box will pop up to select any alternate costumes you have unlocked for that character.
