I tried to assign number pad keys to my side buttons, but did not succeed? (I use German layout keyboard)

As the English version software only recognizes English name of the keys, it will not recognize key names in other language. If that's the case, please change the layout of the keyboard to English in Windows setting: https://www.howtogeek.com/121169/how-to-change-your-keyboard-layout-in-windows-8/