Where can I get Microsoft C 6.00A
You might be able to buy it legally here:
http://www.emsps.com/oldtools/msc.htm Probably any version >= 6.0 will compile the code fine.
What do I need to change so that KSM lets me input notes on the computer keyboard?
Answering this basically requires me to do all the work for you. Currently, KSM uses getch() and kbhit() for input, which does not allow simultaneous key input (i.e. chords would not be possible). So the first thing I do would be to write an interrupt handler for the keyboard. If you don't know what an interrupt handler is, then have fun googling all day. I don't have time to teach it.