For starters I would suggest reading the stickies at the top of the RealFlight section of the forum.
First, Sticky: Frequently Asked Questions - RF7:
https://forums.realflight.com/showthread.php?t=30570
This is your RealFlight 7.5 Cheat Sheet. It has valuable information, that anyone using RealFlight, Should Read.
It can answer Questions, you haven't even Though,t to ask yet.
Next, Sticky: Field Radio Interface Instructions:
https://forums.realflight.com/showthread.php?t=30487
This Contains General, and Specific information, on setting up field radios.
Your DX8 settup, is Specifically Detailed.
In the Main Menu (menu at the top of the program) Click Help.
The RealFlight Manual, along with more helpful information, can be found there.