I've got the same phone and use mp3's for my ring tones. The way I do it is I put the memory card from the phone into my card reader then copy my chosen mp3 to it. Put the card back into the phone, go to 'media' then 'sounds'. Scroll down the the mp3 track thats on the memory card press the middle menu button, then scroll down to 'move' and press 'Select'. Highlight 'Phone' then click 'Select' this will move the mp3 from the card to the phone memory. Find the ringtone folder and the mp3 will be in there.
Hope this helps.