How can i set a ringtone for text messages on my blackjack II?

I've looked for answers to this question and everyone has said that you can do it but i can't figure out how. I've tried moving the file, searching for it, setting it as my ringtone first, everything.. and I still can't seem to make it work. Somehow i did it once before but i don't remember how.. it just sort of happened. help please?