Getting to the terminal is easy: while you're sitting in the chair in the main menu, hit the left and right triggers on your controller a few times. L, R, L, R, L, R, L, R, L, R! After a few back and forths, Mason will rise up, free himself from his shackles, and you'll be able to wander around the room. From there, the only thing to do is either sit back down or head to the back of the room
So the obvious things to do in the Black Ops terminal are the DOA and ZORK commands. DOA unlocks Dead Ops Arcade, a cool dual-joystick shooter that you can find on the zombies menu once you've got it open. ZORK, as you might have guessed, launches the classic text adventure, Zork. Yeah, dude. Zork!
But there's more to do in the terminal that just unlock that stuff. Typing in "3arc Intel" will let you open up all of the intel documents, though that'll prevent you from earning the trophy or achievement tied to unlocking them naturally. And you can also read Mason's e-mail by launching the mail client (by typing "mail" into the terminal).
The "who" command gives you a list of accounts on the system and the "login" command lets you attempt to login as another user. This is where it starts to get interesting, since each user has his own e-mail inbox and some extra concept art to see. Most of the passwords I've figured out so far have been relatively straightforward, but they aren't really found in the game. Instead, a little Wikipedia scouring turned up passwords for multiple presidents and a Manhattan Project head.
There's probably more to do here--I still haven't figured out how to get on the remote Dreamland server run by Majestic-12--but here are a few passwords to get you going.
l: jfkennedy
p: lancer
l: rnixon
p: checkers
l: lbjohnson
p: ladybird
l: vbush
p: manhattan
And if you ever want to get back into Mason's account, amason's password is the most obvious of all... it's password.
Let me know if you find anything else, and good luck!