Are you trying to use an .mp3 file to make an audio CD that has .cda extensions, or just burning the .mp3 to the disc?
If you want to make an .mp3 disc, try using the jukebox setting instead of the audio cd. If you want your .mp3s converted back to a normal audio CD format, Nero should do it just fine, as will iTunes and some others.
Also, is some situations, it makes a difference whether you drag and drop the files or add them via the add files dialogue. So whichever way you are using, try the other way.