Ohh ok ! i got it !
Thank you so much Pbacot and JQL for the help !

May i ask if this works on Sketchup 2015(i am using window 10)?
I'm a total noob to sketchup , I've tried putting the rb file into sketchup tool folder, and it shows some error when i start up my sketchup ,so i downloaded the rbz file at extension warehouse.

I try to test the plugin with a simple cube.
Then i select everything?->extension->Normal map maker-> export
After its done , i went check the file , it only shows a purple square cube without the depths of other colors like the youtube tutorial shown.

I did everything i can , things like : changing to parallel view/ top view first before activiating the normal map maker or applying material and then try again.
It didn't work. did i do anything wrong or ?

Please help /_\ ....

Thanks all..

Vic.