Before starting RealFlight, you need to bind the USB dongle (WS2000, I assume) to your DXS, just like you would bind an airplane receiver to it. For the WS2000, you hold down the bind button while you plug it into the USB port; that gets it into bind mode. Then you can turn on the DXS while holding down the bind button on it.
In RF, you need to go to Simulation menu > Select Controller...
From the list of choices, choose "Spektrum Receiver (DXS)".
Then click the "Calibrate" button.
Before beginning the calibration process, center the sticks (push the throttle up) and also center all three-position switches and the knob if it has one. Then when you proceed to move everything through its complete range of motion, move the sticks in the shape of plus signs (+), not big circles around the edge. Move all switches to all of their possible positions, and move the knob fully back and forth if it has one.