Thank you,

the question was asked a long time ago, but the topic remains relevant, of course.
There are now two articles on this subject that I can recommend additionally.

on ui_control explained

set_engine par explained

{Hook up knobs to engine control}
on ui_control($attack)
	message("attack")
	set_engine_par($ENGINE_PAR_ATTACK,$attack,0,find_mod(0,"ENV_AHDSR"),-1)
end on
on ui_control($decay)
	message("decay")
	set_engine_par($ENGINE_PAR_DECAY,$decay,0,find_mod(0,"ENV_AHDSR"),-1)
end on
on ui_control($sustain)
	message("sustain")
	set_engine_par($ENGINE_PAR_SUSTAIN,$sustain,0,find_mod(0,"ENV_AHDSR"),-1)
end on
on ui_control($release)
	message("release")
	set_engine_par($ENGINE_PAR_RELEASE,$release,0,find_mod(0,"ENV_AHDSR"),-1)
end on

Thx!

Yes, you can use get_engine_par_disp() to get pretty much all of the values directly from the Kontakt engine. This automatically shows the right values and units, with no recalculation required. This also adds the correct unit like Hz, ms, dB,…

EDIT:
for displaying the values inside your UI create a label and change the label’s text via

@text := get_engine_par_disp([parameter],[group],[slot],[generic])
set_control_par_str(get_ui_id(@your_label_name),$CONTROL_PAR_TEXT,@text)

you can also refer to the custom ui tutorial and to the mouse release tutorial