There is an application called "Ford M serial calc.rar" which you can download.  Enter your serial # and it gives you the code.  It will be a 4 digit code.  Each digit is entered by pressing the associated # button that many times.  For example to enter 6753, press 1 6x, then press 2 7x, then press 3 5x, then press 4 3x,  and finally press the 5 button once to enter the code.  Note: you'll have to get your code #, 6753 is just an example.
I found where to download it by Googling "Ford M serial calc.rar" and download together.
                
 Neil
                
 June 2009