Basically PCK is the data file but the information are stored in other files (PCKX I guess) that may be encrypted.
You can try uploading models.pckx but I'm quite sure it's the same like in Swordsman Online.
The good news is that you can extract the files from PCK using offzip -a but they will be without name and the non-compressed files will be not extracted.
It's all ok, you just need to specify the full path of offzip.exe. If you drag'n'drop offzip.exe in the Command Prompt, the path will be generated automatically.