Your question must have meant how to unlock zombies maps not mode. You unlock Dead Ops by getting up from the torture chair and go to the computer and type in some stuff... Search on google and you will get a more detail answer... The Pentagon (Also known as "Five") is unlocked by finishing the campaign.
Burger, you are correct. Originally the campaign had to be completed or W@W before being able to play zombies. Later, they patched it and all you had to do was download the update to get Zombies on the main menu without beating the campaign. Unfortunately, I have no info on how this will work for Black Ops