Ok...so, here we go.  I'm new to midi, but aggressively learning so one day I might be a resource.
I have X3 with a Focusrite 18i8 interface and a Yamaha DX21 synth hooked up to the 18i8 midi ports.  My computer will not recognize the DX21 as a device.  Is my problem...
 
a. The DX21 should be hooked up to my computer through a separate USB-MIDI converter.
b. I have something incorrectly set up on X3.
c. I should wait and try again when my A800-Pro arrives in the mail?
d. Something needs to be done with my computer.
e. The synth is too old.
f. Other...
 
I can't install the USB-MIDI driver I got from Yamaha because my computer won't even see that I have a device hooked up.  I think I'm hitting some of the basic beginner midi questions, but I'm all open to learn.  Thanks for the help.

Your DAW will only see the MIDI ports in the 18i18, it will have no idea what is connected to the 18i18 and it should not matter as long as the device is MIDI capable.

Check to make sure you haven't got the MIDI cables reversed, and that MIDI OUT on the synth goes to MIDI IN on the interface. I know, it's obvious, but I can't tell you how many times I've gotten it backwards while re-setting up my synth after bringing it back from a gig.

scook
Your DAW will only see the MIDI ports in the 18i18, it will have no idea what is connected to the 18i18 and it should not matter as long as the device is MIDI capable.

scook is right on with his reply IMO. I use a DX7s (2nd Gen DX7/single timbre version of the DX7II, circa about 1987) as my MIDI keyboard and X3 has no idea what it is. It just knows that it is getting a proper MIDI signal, which it dutifully and error-free passes on to my X3 soft synths - including velocity, aftertouch, program changes, etc.
 
edit: I also use to own a DX100 and have at least 1 if not several patch tapes for the DX21/100. Amazing that we used to have to load voices onto some of these things via cassette tape - and rejoiced greatly when they started putting floppy drive in them instead of tapes or ROM cartridges...

Another maybe is that it might not have a midi out.  My old yama drum machine (RX something) was the cheaper version.  Years after I bought it I found it did not have midi out - it wasn't set up to output the midi patterns while the more expensive version (w/ individual outs) did.  The 21 was a cheaper dx synth, it may be restricted the same way.
 
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