When i flashed my K810. i went threw these steps with XS++

1: select main_generic_etc.....
2: select fs_Europe_etc.....

Than flash it.
Wait until "successfull"
Disconnect phone.
Remove and replace battery
connect to XS++ again
Go to "FSX" --> "start FSX"
wait until list completes
goto --> tpa\preset\system\custom
and just simply add the custom files right in there. works for me.

Good Luck!

Best Regards
twinn_aces