As said above just a regular CD-R. Burn it like you would burn data like a word file or something like that. Depending on how long the songs are = the bigger the files, determines how many songs you can get on each CD. I have got about 120 to 150 songs on a disc. You can also group the songs together in folder. I put all the songs by one artist in the same folder or a bunch of songs by artist names A-F. If you have them in folder like that you can make it play all the songs in a folder then start playing songs in the next folder or you can let it play all the songs one after another(there is a button you push to switch from all play to song by folder I think the button says "DIR" don't hold me to that. It tells you a lot of info in your owners manual.
The Machs are pretty good at reading most MP3's. They don't seem to mind VBR's, but will choke on one's bigger than 10 meg. At least that's where mine seemed to have issues. The unit doesn't read the whole file (like most aftermarket units), so I always wondered why it couldn't digest large MP3's. I believe max song capacity is like 250 or 254, something like that.
A way of recording MP3's so the bit rate (complexity of sound signature) varies according to the music. IE, less data required if the music has less notes. This is different than Constant Bit rate, which stays the same even if there is no sound.