I'm not understanding what you mean by applying an ".obp" file to a primitive. I gather that you've downloaded some ".obp" files which contain materials that you wish to transfer over (apply) to a primitive like a sphere or a square. If this is correct, then all you need do is go to the "create" menu and click the triangle and click import and bring your "obp" files into the folder you want. Once there, you can then bring the "obp" into your scene, click the "m" by the wiremesh and copy it's material to memory, then bring in your primitive and paste the material to that. If you wish to save the material by itself then while still in the materials lab, just click the triangle by the upper right of the material display window and click on "Add" to whichever folder you want it to be in. Hope this helps.