How do I set GR50 for MIDI In and Out?

In the GR-50 manual it says that in order for MIDI In and Out to 
work, the synth must be set to "Poly in OMNI OFF."

The manual is not exactly clear on how to do this.  I can find some 
MIDI settings by pressing EXT, and I think I have it set to Poly 
rather than Mono (though it seems that Mono would be prefereable, 
sending each string on a different channel), but I don't see anything 
anywhere called OMNI.  It's also not very clear how to assign strings 
to channels.

I'm a MIDI noob, but I've been around technology a long time and I'm 
not afraid of manuals.  But this one is more than a little obtuse.

How do I set this thing up to speak MIDI to another device??