I have no idea what it is, but that's the part (Realflight Transmitter Interface Adapter Cords (RFL1015))
that Gordonski suggested and he seems to know what it is. Personally I too would go with the WS2000 Dongle (as an easy solution).
If you look at the back of the InterlinkDX, it is an audio plug receptacle. In the past, I have used mono plug wired connections (just like buddy boxing), but I have had a stereo cord work too I believe. You can get those at 7-11.