Here is how I did it. Go into your contacts. Choose the contact that you want a custom message tone. Edit that contact. At the bottom choose to add additional fields. You can choose to add message ringtone there. When you save from there, you will have that option to change the ringtone. save the contact and you are done.
This did not work on my phone (LG Optimus Fuel) when I choose to add additional fields this is what it gives me: Organization, IM, Notes, Nickname, Website, Events, and Relationship. There is only a "ringtone" space on each contact. And now I have no idea what I have done, but when I go to settings, sound, my ringtone and notification is greyed out and I can't even change them anymore! This is my first "smart" phone and it sure is making me feel stupid. I use zedge as well and I went into notification and grabbed one I liked and thought it would set it as a notification and it set it a ringtone. Help!!