Yamaha DTXpress/DTXplorer/DTXtreme

Hello all,

as I'm a bloody MIDI beginner, I would ask you all to give me some
assistance how I could expand my DTXpress with MIDI.

First of all I would like to connect my really old Yamaha DX-21
Synthie with DTXpress in order to play the DTX sounds via the
Keyboard of the DX-21.
What do I have to do? Which device has to send, which one has to
receive and what settings have to be made?
As I said in the beginning, I have no idea about MIDI, never worked
with it, I only know that musical devices can somehow communicate...

Can somebody help?

Thanks in Advance!

Mark