Zombie Mode +5 Rounds:
When the Shadowman appears on the staircase in front of you at the beginning of the first round in Zombies every headshot you get will give you +1500 points and send you ahead 5 rounds.
Unlock Dead Ops Arcade 2:
Access your safe house at the Campaign screen and go to the Data Vault terminal. Then select the small octogon icon that is at the top left corner to be taken to Dead Ops Arcade 2.
Unlock Nightmare Mode Without Beating Campaign:
Go above the emblems icon when you are in the Data vault and hold ALL the triggers. Then press Square and Triangle so the keyboard console appears and enter the password 'Dreamland' (without the quotes).