I realize it has been a while since you posted this message, but I have a DX21 Owner's Manual and can make a copy of it for you if you still need one.
Let me know.
Also, I am trying to figure out how to hook my DX21 up to my composing software, Music Time (not the greatest, I know, but I'm on a budget). If you have any tips on software and MIDI transmission, etc., I would greatly appreciate it.
ThomasB