Zip files are little problem with MartinC's utilities Maconverter and Macinstaller. (Of course Mac users also need a recent version of Stuffit Expander to expand the zip files to begin with.) Maconverter takes care of fixing the file signatures, converting .rsr and .pcf files. Macinstaller puts the files in the designated place in the Runtime directory, as well as fixing internal file references so Poser knows where the textures are without having to ask all the time. Both utilities are available in the Freestuff section here, for any new Mac users reading this.