Motorola V710

Can someone tell me how (if possible) to use an mp3 ringtone on my V710 through Verizon? I have downloaded mp3 ringtones from Sony Musicbox which is available through Get It Now. The ringtone plays fine if I just play it as a media file. However, when I assign it as a ringtone, it will not work. Instead, I just get the generic ringer. I have done nothing to the phone and have added nothing. Please exlain (in detail and laymen's terms) what I need to do to be able to have mp3 ringtones. Thanks.