Mike:
I don't have a G91A personally, but will offer this advice under the assumption it's the same protocol as a 565 or 568. If so, there will be several digital outputs which are numbered. Typically #1-2 feeds your L/R mains in a daisy chain. An M-cable goes from the G to the Master of the two speakers, and then an S-cable goes from the Master to the Slave of the two speakers. Then, an M-cable goes from digital out #3-4 to the center. Both of your M-cable control plugs should also plug into the back of your G (again I assume there are multiple instances of this as there are on the 565.
Of course, you need to configure the processor accordingly, using the M-config software which is pretty straight forward. If you don't have a computer (and a serial cable), I think you can do this using the soft keys on the front of the unit (again, assuming 565/568-like functionality). This requires stepping through menus in a very particular sequence which the manual should expose.
Good luck!