To set a Mac lock screen message, head to System Preferences > Security & Privacy > General. Click the padlock icon in the lower-left section of the screen and authenticate as an administrative user.
Find and check the box “Show a message when the screen is locked” and then clickSet Lock Message.
In the text box that appears, type any information you wish to help return your Mac to you, such as a phone number, address, or email address. We’ve also found that the lock screen message is a handy way to quickly identify identical hardware. AtTekRevue, for example, we have two 15-inch MacBook Pros that look the same but run different software. We use the Mac lock screen message to label the first system “Alpha” and the second “Beta,” so that we can quickly tell which system we’ve got in hand.
You can enter as much text as you want in the Lock Message box. On the Mac lock screen, OS X will display the top three lines by default, with a scroll bar to view additional text. If you wish to enter line breaks, press Control-Enter. Otherwise, the text will format as a single paragraph.
Once you’ve set your message, log out of your user account or lock your screen to see it.
While our screenshots demonstrated this process using OS X Mavericks, users can set lock screen messages in any version of OS X starting with 10.7 Lion. To disable your Mac’s lock screen message, head back to the Security & Privacy preference pane and uncheck the box referenced above.