Retractable Landing gear configuration

Hello Everyone,
I have a problem with landing gear on px4. I followed this link.

I have mapped the RC_MAP_GEAR_SW parameter to the RC channel 9.
All I want is to retract and undo it using RC channel 9.

I have put the following content in mixer file located at /fs/microsd/etc/mixers.gear.mix file:

M: 1
O:      10000  10000      0 -10000  10000
S: 0 7  10000  10000      0 -10000  10000

I took this file from given link. Does it mean that the RC channel 7 (second column of S) map to pwm output 1 (value of M) ? I could not quite understand it clearly. If I map RC channel 9 to
RC_MAP_GEAR_SW, then do I have to put 9 in place of 7?

I have connected the cable for landing gear to auxiliary output number 5 to pixhawk. I am not sure how to specify the output to PWM number 5 in the mixer file.

And the link mentions about loading the mixer file as:
mixer load /dev/px4fmu /etc/mixers/gear.mix

I have tried to execute this command from nsh. I have also tried:
mixer load /dev/px4fmu /fs/microsd/etc/mixers/gear.mix

But I get unable to load mixer file. Am I doing right? Please suggest me the correct way.

Thank you.