well im only used to using pcs. when you download music on itunes, see if it saves in "my music" (it probably should automatically, check your settings on itunes. theres probably an option.). if it does, then open up "my music" and "my computer" at the same time. once your at my comp., go to the MMC/SD card slot. then, simply drag the music files from "my music" to the SD card file, unplug your card when youre finished, and put it back into your phone.