You actually want four snap roll mixes. The Futaba 7C has the capability to set only one. You cannot set up snap roll mixes for left and right, positive and negative snaps. Pick only one. Other types of snaps will have to be done in the conventional manner: move the sticks as necessary to get the snap in the desired direction.

To change the numbers from positive to negative, you will just rotate the dial to set the number you desire.

If you want more snap roll mixes, you need "more" transmitter. The Futaba 10C is the "smallest" transmitter that will have the four snap roll options. Of course, the 12 FG, 12 Z, and 14 MZ also can do this.