Upload to Poser is an incorrect term, you are installing your content into Poser; upload is the complete opposite of download, you would be uploading your files to a server, cloud, storage, or website.  It is proper protocol to use the correct terminology to convey exactly what you mean.

Anyway, what type of model are you attempting to install into Poser?  Is it Poser content you've downloaded from a site or a model that is not exclusive to Poser?  Everything depends on the type of content packaged within the zip file.  If it is exclusive to Poser then the zip file should have everything separated into the necessary folder hierarchy for Poser to immediately reveal it in its library.  If it is not for Poser then we are dealing with a different file format that may need to be imported into Poser.  There is a difference.  

How are you installing your content?  Do you have single or multiple Runtime folders?  Do you have a link for one of the files you've downloaded?  This will help us identify what type of content you have and exactly where to find it.  And, how proficient are you with Poser?  This is important because we need to know how technical we can get in explaining details.