Providing that the zip contains the correct folder structure, and most do, you need to install into the folder ABOVE runtime.

If your Poser installation is at C:Program FilesSmith MicroPoser 8
Then extract the zip to C:Program FilesSmith MicroPoser 8

If you are using a remote Poser library such as C:Docunents and SettingsAll UsersDocumentsPoser 8 Content
Then extract the zip to C:Docunents and SettingsAll UsersDocumentsPoser 8 Content

If using Windows you can explor the zip to reveal the Runtime folder. Then drag that to your Poser folder, or remote library Poser folder.

Or you can use this free utility:-
http://www.philc.net/unzip.php

Hope that helps.