Wow, DigiTrax manuals are written in a complicated and verbose fashion.
I presume you have tried setting CV 49 to 104?
Dimming is hard to achieve on LED lighting, because photon output proportional to current, and it's hard for devices like an interrupt controller to vary current. They can vary the pulse width of the applied voltage, but 'persistence of vision' causes us to see little difference.
With that in mind I occasionally use the primary function to pull a small current (via large resistor) and an additional function wired in parallel to pull a larger current current via a small resistor. Primary function on will produce a dim light, turning the additional function on and off will induce a bright and dim light correspondingly, turning both functions off turns off the light.