Looks to me like futaba forgot to include the where to plug in part out of the manual. It will be either 5,6,7.
On my 9c I use 6 for aux elevator, 7 for aux aileron. the 7c uses a 6 as a default for aileron so I would look at
5, 7. after activating the mix, and setting ailerons to 0, plug into either 5, or 7 and move the elevator stick.

After a bit of searching it looks like the default channels are 1,6 for ailerons, 2,5 for elevators.