The G3 or G4 sim comes with a few couple of adapters and a cord that looks like a mono mini-jack audio cable. First, create another model on the DX7 as "airplane mode" and leave the DX7 powered off. And keep the simulator closed out (not running).
Plug in the simulator transmitter USB plug into the computer.
Plug the square connector to the back of the simulator transmitter.
Plug the mono mini-jack cord into the other end of the square connector.
Plug the last end of the mono mini-jack cord into the back of the DX7.
The DX7 will power on automatically.
Do not flip the power-on switch; leave it off.
Then start up the simulator software, select the transmitter "Interlink 8 channel" from the "Controller" menu, and proceed to calibrate the transmitter.
Lastly, you will have to map the channels from the "Controller" menu. You may have to go back and forth to the channel mapping menu to get it right and test each setting you make.
Then it will fly the heli perfectly!
Note: The same mono mini-jack cord can be used to buddy-box the DX7.