another way to unlock the music you legally bought it to burn it to a CD and rip it back off.. if you are doing individual songs you may have to re-enter that tag info.. if you buy an entire album, you can burn it onto a CD (in the correct order), when you put it back in it will recognize the songs and fill in the tags for you..
i am not sure what happens if you burn an "mp3 CD", that might work better, i always burn an "audio CD"
for the future you can set iTunes up to rip your CDs to mp3s instead of ACC.. fortunately that is how i did mine and they play on the RNS-E just fine, though the tag info is incomplete..
also you can use a CD-RW so you dont waste CD-Rs..
I've been running into the same issue every since I got my RS 4 this spring.
I was having a *blast* converting all the files into MP3, copying them onto the SD card, and then deleting the MP3s from my iTunes library, but I thought there had to be a faster way of doing it all, let alone creating M3U files. It doesn't seem like there's anything out there that really does just this, so I taught myself a little AppleScript and wrote an app that does this in iTunes.
NOTE: This only works if you have a Mac and are running iTunes, of course.
It basically creates a library of folders for each artist in whatever location you choose and then also creates playlist files for the playlist you exported (if you chose to) and a playlist for each Genre of your songs. You can save them in whichever directory, provided it's all on the memory stick.
I've tested this a little bit in my car, and it works pretty well. It can take a while to run when you're converting, but hopefully it's faster than the alternative. Let me know what you think!