Having the ability to change the text on Mac OS X’s Login window is not an explicit feature of Apple’s operating system, it is however possible. Some of the reasons you may want to add text to your Mac’s login window are: security, lost/found and identification. Adding text to the login window is easy, simply add a key and value to a .plist file and logout. 

First you’ll want to navigate in Finder to the following location: /Library/Preferences/ (the Library folder in your hard drive’s root, not your home folder).

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Once there, find and open the file com.apple.loginwindow.plist in a text editor like TextWrangler.

Because this file, com.apple.loginwindow.plist is outside of your home folder and effects all users on your system, you must have administrator access on the computer you wish to modify.

Make a backup copy of your original com.apple.loginwindow.plist incase something goes wrong in any of the following steps.

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Insert into com.apple.loginwindow.plist somewhere after the <dict> and before the </dict> the italic text exactly how it is shown here: <key>LoginwindowText</key> Once that has been added, directly below it on a new line enter <string>Your Custom Text Here</string> Replace the Your Custom Text Here (red) with your message. Image: The text you will be adding is highlighted in yellow (the end of the line was omitted).

Save the file and logout to test your new login window text. TextWrangler or any other text editing program will ask for an administrator password when you attempt to save this file.

To remove the text you just added, simply remove the two lines of text you added.