Why can't I click the button to add a new user?

Question

I want to ask you something. I tried to create a new user account on Ubuntu 12.04, but, when I am in the "User Accounts" window and I go left-down (where the "+" sign is), I can't click on it. It's not click-able.

Answer 1

You must click on Unlock button in the upper-right corner first, enter your password, and then you can add/modify users.

Notice that after that, the + is now black and can be clicked, instead of gray: