How do i make material expression asset ?

There is this document :

But i cant seem to find where i can add something that gives me this kind of work ground .

What do you mean by you can not find it? The page that you linked shows what the various material nodes do but to get that functionality you need to make the network your self.

But where do i start ? I don’t even find a gui that looks like that ( like the old UDK material editor )