Hi R/Ckrazy,
Unfortunately, the Reflex demo that can be downloaded has limitations, for obvious reasons. There's no way of controlling the demo with a (analog, digital, or USB) joystick. I tried them all.
But, you can still use the mouse to control two of the channels.
This is what I do: First initiate a flight. Then press F8 to bring up the "Channel Display" control window. Then move any two sliders to control the plane/heli.

NOTE THAT ONLY THE FIRST TWO SLIDERS YOU MOVE WILL BE USABLE

The full version comes with a custom USB cable that connects to your transmitter (transmitter not included). And this cable-transmitter combination works with Reflex but not with RF G2, FMS, or DB RCFS2001.

- Juny