OS X Lion: Display a message in the login window
You can add a short message to the login window that is displayed when the computer is locked. The message can be anything appropriate to users attempting to log in or for example, guest log in information. The message can also act as a virtual engraving, to provide contact information for a misplaced computer.
- Choose Apple menu > System Preferences, click Security & Privacy, and then click General.
- Click the lock icon to unlock it, and then type an administrator name and password.
- Click “Show a message when the screen is locked.”
- Type the message you want to display in the login window.