First question must be, Do you run conventional and DCS? If you run conventional, and want to use only the remote, the only way I know of to use the FIXED channels and vary the output voltage is to use a Z4000 transformer with a Z4000 receiver, which enables the DCS remote to control the output voltage from the Z4000. If you don't run conventional, then you simply provide a power supply to FIXED 1 & 2, that provides a voltage of around 16-18 volts. Higher voltages will shorten bulb lives in locos & cars.
Do you have Barry's book, which goes into detail about setting up channels? (It has been on sale on Amazon.) I believe this was also covered in the DCS manual, although it's been so many years since I read it and set up my layout, I forget what it said.