How can I use the elevator?

[Gonsnakex]

Do I need to destroy all the cameras first?

[MW2K]

You got it.

[Darsiden]

It isnt exactly necessary to break every camera, you do that if you want to explore every inch of the deck.
To unlock elevators, you should destroy CPU nodes, which already takes a good chunk of security level away.

[Shadow]

Getting the security level down to 0 also completely stops respawning on that level (I think), turning it into a safe place to return to to recharge, heal, recycle, etc.