I've just installed Import OBJ with Materials 2.1 and am running into the same problem, with materials being replaced with white/grey. I'm exporting from Poser, and the .obj and .mtl files are in a folder, with a Textures subfolder containing the generated PNG files. I've also tried it with the PNG materials in the same folder at the .obj and .mtl files. Neither works. I'm running Sketchup2015 64 in Yosemite on a 5K Retina iMac if that helps. Any suggestions?