Set material function not show all materials in content folder


Hello,

I follow the speaker configurator tutorial and everything works as shown.
My question is when l set the material function I can’t see all materials in my content folder, such as Megascans surfaces or some of my materials.

I mean l can’t define external material to set material function because many are invisible.

What must l do to see all content folder materials in the Set Material function?

Thank you!