real flight 8

I hesitate to jump in as I'm not familiar with the system, but if you only have the 'Realflight RF-X Wireless Interface' then it will not work with most radios:

"instantly ready to link up your SLT radio (Tactic)"... "Required for non SLT transmitters: P-TACJ2005 - Anylink2 - 2.4GHz Universal Radio Adapter"

The transmitting protocols between the two systems are different, to make the Spektrum work with it you apparently need the Anylink2 - 2.4GHz Universal Radio Adapter which I assume plugs into the trainer port on the back of the transmitter. This is another £38 www.sussex-model-centre.co.uk/anylink-2-2-4ghz-radio-adapter

I can't help thinking this is a lot of money for the ease of a wireless setup.

Please double check my conclusions I've got a cold and made more than my usual number of mistakes in the last couple of days.

If Chris is correct and you don't already have the Anylink adapter, I'd suggest getting a refund and buy a cable to join your DX8 to the PC. A wired connection is not a problem for a flight sim where you're not moving around much anyway. And wired could be preferable for the solid link and lower power consumption.