Unlike the 146, the RJ doesn't have seperate positions for flight idle and ground idle, it just has an "IDLE" position (labelled on the pedestal).
Perhaps you are referring to the FUEL ON / FUEL OFF positions? In which case the throttles can be moved between those two positions either by clicking the latches on the rear face of each throttle, or by using the SET ENGINE X FUEL VALVE control assignments (where "X" is the engine number).
Mark - Just Flight