Akuma:

First go to SPIRAL for three seconds, then without stopping go in this order:
SILVER SAMUARI, PSYLOCKE, COLOSSUS, ICEMAN, COLOSSUS, CYCLOPS, WOLVERNIE, OMEGA RED, SILVER SAMUARI
Now while on SILVER SAMUARI wait for three seconds then press short kick, fierce punch and roundhouse.

Get the Bosses (PC Only):

This code will let you use Magneto and Juggernaut. To do it, simply go to the Option menu by pressing f10 and then go into the configuration menu. Once there, type in the word SPAM . After typing in the code, go into system and you will see a new option called BOSS SELECT. Enable this option and then save the new settings. When saved, start a normal game and select a fighter and block mode. Now just before the battle loads hold all three punches down (if you want to use Juggernaut) OR hold all 3 kicks down (if you want to use Magneto.) Continue to hold down the 3 punches or 3 kicks until the battle finishes loading. You should now be able to use your new boss character.