If using WinZip, a file can be right-clicked to look at File Properties to see where it's installing.  Typos in names send things to wrong places that Poser won't see. Some zipped files aren't set up to unzip to the different folders and will dump everything into the location it's unzipping to, requiring manually putting everything in the correct Poser folders.

When unzipping, you should be unzipping to the main Poser directory (if you haven't created addiitonal runtime directories). Unzipping to a temporary directory, as mentioned above, is another way to get used to what goes where.  That means the directory you want to unzip to is Poser 6 (or 5 or whatever version you have).  If you unzip to any lower directory, it will create a Runtime folder under that directory folder and Poser won't find it.

It takes a while to get used to everything. I'd imagine if Poser's creators had ever imagined that there would be so much content created for the program that they'd have indexed the library differently, but they're now stuck with what they set up years ago.