Android #16: In your second run of DU with Krillen, after the fight with Frieza, two red dots will appear. Go to the one on the right, and you will enter a scene with a frog. Continue, and your story willl progress to the Android Saga rather than finishing. When this section starts, go to the area near the world tournament, and search for a "???". You will find Android #16, but he will be broken. Now go to West City, and you will unlock him.
Android #17: Defeat Android 17 in Piccolo's DU.
Android #18: Finish Krillen's DU.
Dr. Gero: Defeat Dr. Gero in Yamcha's DU.
Cell: Defeat Cell in Teen Gohans DU.
Cell Jr: Unlocked in Dragon Arena, Piccolo must break in. You will need to fight a character level 66-70 for this random event to happen, and your fighter must be at a lower level than your competitor. Your character will need to be at least a level 33 to unlock fights against level 66 opponents. You will see an extra scene before the loading scene if he has broken in. If you don't get this, you can simply quit and return to the character select scene and try again until it happens. Beat Cell Jr to unlock him. As this is random, it could take any number of attempts to happen, but will eventually.