the .obz file is a compressed .obj. - don't just rename it. It can be unpacked using a program such as winzip (I'm not sure what the mac equivalent is - does stuffit deal with zip files as well as .sit?) The other option would be to use the python script within poser which allwos you to decompress all the files in a given folder/runtime. Be warned, however, that if you have a large compressed runtime, it can turn into a monster when decompressed! 

jonthecelt