The MX-20 works Great for DJI Wookong or Naza Products.

The one thing that is a bit confusing is how to set up your three position switch to work correctly with normal flying and Fail Safe.
first start by assigning a Channel for the switch you want to use,

on the MX20
hit  SET
Scroll down to Control Adjust
hit set
then select Input 5 for the GPS, ALT Manual switch on the DJI system.
then move the thee position switch you want to use.
Starting off with the switch in the desired Off position.
if it is not now just do it again.

I use Switch 7 CTRL10
then  you should see C10 pop up on the screen next to Input 5
now hit esc
With the DJI Assistant pulled up and connected to your Controller,
go to the TX Cali page
under Control Switch U
you will see the bar graph on the bottom of the page.

First,throw the switch to see if the slider moves.
If it does not make sure you have the correct channel assigned to the switch and the correct connections on the DJI unit, start back on step one.

once you get it to move,

on the MX-20
hit Set
now go to Servo Adjustment
hit Set
Scroll down to S5
hit set, and move over to the 0% 100% or 100%
By adjusting these settings by moving over to them and hitting set then
using the up and down arrows, you should see the slider on the
DJI assistant move,
you want the off position of the switch to light up the M
the mid position of the switch to light up the A
and the on position on the switch to light up the GPS.
keep adjusting untill it hits these spots,
something like
5% 83% 86% works for me

ok throw the switch a few times and make sure it does what you want..

Now to set up the Fail safe,

Esc out of the page you are on for now and reset everything,
Power on and off the Kopter and the radio,
go back in and verify your switch is still correctly assigned and working,

now on the MX-20
Hit set and

go back to the Servo adjustment page,

Write down the first three percentages you have adjusted to get the switch to hit the positions.

Set your switch to the center position and then adjust the first percentage
until the slider in the DJI assistant moves to a position that says failsafe.

 about  36% worked for me, It may be -36%
now esc out,

On the MX-20 Scroll until you find
Fail-Safe-adjust
leaving your SW7 ctrl10 in the mid position.
now hit set on Fail-Safe-Adjust
then using the arrow buttons,
move the dot over until it is over ch 5
hit set to move that slider to the Pos setting
now using either up and down arrows move over to the STO
once you hit set it will save the current position of the switch using Ch 5 and use that for failsafe.
Hit STO
ok
so now you can test,
watching the DJI tool again
move the switch to the off position and don't worry it doesn't go to M for now,
but turn off your radio.
you should see it go the Failsafe position. If not try and try again..
ok so now
just go back in to Servo adjust and set it back to the numbers
you wrote down from when you had the switch setup to go to M, A, and GPS
Put those numbers back, you should have only had to change the first one.

 so now test again,
throw the switch and make sure you see it hit the spots on the DJI Assisitant.
and when you turn off your radio you should see it go to Fail Safe..
Woo Hooo!!!
hope this helps someone, it took me a bit..