The command "MM" means "alt M". In fact, any command starting with "M" means an alt key. Some keyboards gobble up the alt key: in my case on the Macbook Pro Retina, โescโ followed by โMโ is what worked. More details here .
However, since your screenshot shows [DOS Format] as the current selection, the key combination you really want is "alt D" (or "esc" followed by "D"). This will disable [DOS Format] so that you save the file with the default line termination character (ASCII 0xA). Turning on [Mac Format] will just give you a new set of problems.
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