This scenario applies to my 2005 SL500 but it should work fine if you COMAND has the MP3 CD soft-key. I've read all the posts regarding the making or attempts there of, an MP3 CD for the COMAND System. Here's what I did to make numerous MP3 CD that play great in my car.
1. Downloaded iTunes from the ITunes web site into my Windows XP machine.
2. Made a "Playlist" called SL500 (name yours whatever you wish) in iTunes.
3. Imported the songs I wanted on the MP3 CD into iTunes Library from My Music.
4. Converted the songs if needed into MP3 (AAC) files using iTunes.
5. Dragged the converted files into the Playlist called SL500. Make sure to put them in whatever order you want them to play or iTunes will burn them as you entered them. I used a standard 700mb CD which will hold about 150 songs.
6. In iTunes, click Edit, Preferences, Advanced, Burning and select "MP3 CD". You do not have to burn at a slow speed. Choose Preferred Speed "Maximun Possible". You DO NOT have to remove the ID tags as others have suggested.
7. Insert a blank CD into your CD/RW drive and click "Burn MP3 CD".
8. When burn is complete, insert into the top slot on your COMAND system and let it load the songs. It will take a variable amount of time to load depending on the amount of songs you burned to the MP3 CD.
9. The COMAND system will start playing the MP3 CD once the songs are loaded. Use the soft-keys to select any options you want to use.
Easy as it can get. Have fun.