It can't be done as far as I'm aware, certainly not without root access.
How are you setting the ringtone? And when you say the ringtone isn't there, do you mean it's defaulted to the standard ringtone, or the MP3 is gone from the card?
If you create a folder on the SD Card called Ringtones and place the MP3 in there, it should then permanently appear in the list of items you can select as a ringtone.