On the DX7 you activate flaperons from the first menu you get when you switch on with the two buttons pressed. Use Aileron and Aux 1, Depending on your set up you may still need to reverse the flap direction.

Also, there is an option on the second menu level (press both buttons with Tx turned on) to set up the flap switch. Play around with the +/- percentages to see what this does

Terry