Converting ACC file to MP3

Hi, this has been asked before and have not found the answer I need.In my itunes library a majority or all are in ACC format(none purchased off itunes) I want to make a MP3 Cd for my car...now I followed the itunes procedure to convert, but when I check the format it did not convert..itunes went through the process of saying converting...took 30 min...but still AAC.
Error came up saying cannot burn CD as not MP3, what am I
missing?? As I am not the sharpest knife in the drawer
but not the dullest either!!!LOL

Thanks