How can you put music files on Sprint's LG 535?

I bought my Sprint LG 535 the other day and am getting around to tryin to figure out the mp3 player. It seems that most of the pages on the web are about the Telus version of the phone, but I'm lookin for help with the Sprint version. I created the mp3/, audio/, and video/ folders within the mini SD card in the media/ file but when I put mini SD card into my computer to try and put mp3 files on it, I keep getting a "disk is write-protected" error with it. I don't exactly know what I'm doing wrong but any help is appreciated.