Nice article about customising your OSX Login Screen
If you want to add a login message to your login window in mountain lion you have to change the right preference. Open up System Preferences in your Applications folder. Open the Security & Privacy preference. Yo will notice that there is a check box that allows you to show a login message. Check this box, then press the button to open up a small editable text space. If the check box is locked out, press the lock in the bottom left corner.