If you already use "OFPmaterials" mod by Kenoxite, put the FPW_matdef.pbo into the OFPmaterial addons folder.

If you don't, put the 3 pbo files (FPW_matdef.pbo, ofp_matdef_addons1.pbo and ofp_matdef_core.pbo) in the FPWmod addons folder.