hey dear i'm not a big cell guru but i can help u hear

the easyest way to get mp3 ringtone in ur l6i is to buy a bluetooth dongel ,install the driver provided and enjoy.(syncronising phone with ur pc will be a tough task do it with calm mind)

but if u really wanna to that with a usb cabel go to google download motorola usb cabel drivers ,download p2k tools from hear imserba,

about usb cabel i would like u to tell i've tried 5 cabels including that of v3i,but the only one worked which was of motoroker (if u'll need i'll provide u the serial no of that cabel right now i'm not having that with me)

try it
have a nice time