The best way to install Poser content is to use the "Install from zip archive ..." command on the File menu. If the zip file is not properly formatted it will normally complain that the file isn't in the expected format. It also makes sure that the bit land in the appropriate folders in the runtime you choose. After installing any new content you'll probably need to either restart Poser or refresh the library to insure that the new items show up.

Alternatively if this is content you've purchased from the store it will show up in the purchase tab of the library. Where you can install it directly into the Purchases runtime library with one click. Again you may need to refresh the library for new content to be searchable.