It’s actually very well explained in the UR User Manual. But to help clarify, the DSPMixFX Application is not used within Cubase and won’t run if Cubase is running. Cubase has it’s own handling routines for the UR built in.
You get to them via the Hardware rack in the mixer. Here too you can access the Rev-x sends as well as the hardware channel strip as well as the Amp simulations.