you have to beat the mission including the secret ones.or just use the cheats

Expert Jo you accept this answer?

No he doesn't because it's too vague! You have to beat the game or go through weapons master mode and beat a few missions and somewhere along the line you will pick up what is known as Arcade (extra) or Vs. Team battle mode (extra) or Two player (extra) or anything else that displays the characters, or lets you control them that contains the (extra) part after it. Hope this helped.