Any where in any runtime you want to put it in. PP2010 will load any kind of poser file from anywhere it finds it.

The official place for material files is in runtime/libraries/materials. I may have the case wrong - just find the folder you have.

I suggest runtime/libraries/materials/BB/BBGlossy.mt5

You could also do what some people (including myself) do. I get my own runtime library in many people's collections.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)