What Bill said is correct: If Windows recognizes a given input device as a game controller (joystick), RealFlight will recognize it as well. (Of course, that doesn't guarantee suitability for the task. For example, you can use an Xbox controller with RealFlight, and if that's all you have and it gets you started in the hobby, do what you've gotta do, but it's certainly not the optimal RealFlight or RC experience.)
Some transmitters have a mode where you can connect them to your computer via USB and they will behave as a joystick so that you can use them in simulators like RealFlight. As far as I'm aware, though, the DX6i does not have that feature. I expect you would need an interface like the Spektrum WS2000. How were you connecting it previously for use with the other sim?
I just assumed he must have a dongle since he said he had used it with another simulator. And I didn't think to mention that 6 channels isn't really enough to be fully functional in RF without using keyboard commands to control the extra channels for some aircraft.