After completing Mission 2: New World in the campaign, you can enter the safehouse at any time. In the safehouse is a data terminal (looks like a big computer screen). Access the terminal and press the secret button in the top left corner (the blue and white logo to the left of the "Data Vault" that looks like an "S") to start Dead Ops Arcade 2. After the intro sequence finishes, you will get the "Silverback In Black" achievement.