Personally I use a DX7, but I just downloaded the DX6i's manual and it's very similar. I may be teaching you to suck eggs here, but I'll include all the small details (which you may already know) just in case. You'll need use two separate servos for your ailerons. Plug one servo into the AIL port of the receiver, and the other into the AUX1 port. Adjust the servo reversing switches as required to get them working the right way. Then get into your radio's setup menu for this model, and change the "wing/tail mix" setting to "dual aileron". Then find your "adjust list" and find "differential dualail rate" and alter the percentage. Start with a small amount (say 5%) and experiment from there.

As Steve said, you probably don't need it... and it's just as easy to use a ail-rudder mix if you have real problems with adverse yaw. Something else to consider - on the DX7 it's called "flaperon mode" and it changes the "flaps" switch to a "flaperons switch". If the DX6i works the same way and your mustang comes with flaps you may be sacrificing them to get differential.