Hi - just tested this out for you.
I do not normally have my Yamaha midi keyboard checked in Preferences>Midi>Devices section, because my soft-synths have WAY better sounds than the on-board ones of my cheesy YPG-235. I bought it because it has 76 notes, and I use it to play mostly piano sounds through Kontakt 5.
OK, like you indicate in your post, when I went to Preferences>Midi>Instruments, I couldn't really do anything.
So, in a thinking it through for a moment moment, I went back to the Devices and checked on my Yamaha keyboard in the Output devices section.
Then I went back to Instruments, and this time I have a BUNCH of stuff listed on the left side, and when I choose one of the lefts and one of the rights, I can then click on define and I get the instrument definitions dialog box.
Give this a shot and please post back to tell me if that addressed this issue for you,
Bob Bone