I installed Terrain Generator in IC8 and the Textures/Materials are all in the  C:\Users\Public\Documents\Reallusion\Reallusion Templates\Media\Material\Nature Terrain Generator like they wanted me to do so in the Instructions. (Default location) But the Plugin can not find them. 
Any Ideas? 

You should put them where your other Reallusion Template Media files are located.

For example, my main Reallusion folder is at F:\Reallusion, so the NTG materials are under: F:\Media\Reallusion\Reallusion Templates\Media\Material\Nature Terrain Generator.

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The Problem was the Content Manager itself, who listed the items as installed but somehow did not install anything of the Textures or materials. At least not in any IClone location. I had to de-install it and install it again. Now it works. No Idea what made that happen...