From the Adjust List menu, scroll down and select either Mix 1 or Mix 2 and enter it. In the menu, scroll to the left of the arrow, push the roller in. Scroll and select Gear. To the right of the arrow, select Elevator. For the U rate, you can leave this at zero since your flaps won't be going up. Enter 40% for the D (down) rate. If you want this mix to always be active, you can switch option set to On. If you want to be able to turn this mix on and off (if you possibly want to see how the plane flies without the mix active) with a switch you can assign this mix to a switch on the transmitter. You can leave Trim INH (inhibited) since you shouldn't be making any trim adjustments to the flaps.
That's generally how you would set up this Flap to Elevator mix. I haven't tried this on a Carbon Cub personally, so you may either need to reverse direction on the Up/Down rates, and you may also find that you want to increase or decrease the percentage of mix to suit your model and flying preferences.
There is a little bit of information that can be found in the manual for the DX6i regarding mixes: http://www.horizonhobby.com/pdf/DX6...