Unless your phone has a mp3 player on it, it cannot use an mp3 file as a ringtone. The most common type of audio file for a ringtone is .mid which is known as a MIDI file.
Based on your phone's specs, it should be able to use a .mp3 as a ringtone, however, your .mp3 file may not be using the right codec.
I would recommend using either GoldWave or Audacity to change the mp3 to a lossless codec.