The content directly downloaded from the Posersoftware site is bundled into the correct folder hierarchy so that decompressing the files into the Library should be completely painless.  However, the Add/Remove Library icons are just for importing established runtime folders that have been externally created by the user.  For example, if I wanted to install all my La Femme content into one exclusive location, I would create a folder entitled La Femme or La Femme Content and unzip all my files into that folder but, again, provided that those zip files contain the correct folder hierarchy as I mentioned before.  Looking into a zipped file, the first folder you should see is Runtime.  Of course, there will be other folders that will not appear in your library but those will contain additional files like textures, geometry, documentation, etc.  

Below is an example of a compressed file that is set up correctly for installation.  As I mentioned above, the primary folder is called Runtime, and I can directly decompress the contents of this file into an external Library runtime, in my case Dawn.  

                                                                             

Now, this is just an example of a manual installation into an external runtime folder.  You can have multiple runtime folders categorized in any way you want, by Characters, by models, by props, by scenes, etc.  This is just to organize your content into some cohesive manner for easy access within the Library.  I have over 100 runtime folders on my hard drive and there are others that can boost the same or higher.  

But you want to add content as quickly as possible so that you can play with the program.  This is how you will easily add content within Poser.  On the upper left-hand corner on File, open the contextual menu, and navigate to 'Install from ZIP Archive...'.   A search browser will open, and navigate to where you downloaded your content on your drive.  Highlight the Zip file and click on the open button.  A new window will appear asking for a location in which to install the content.  It will appear as below:

                                                                                       

You will be installing a runtime so if you open the contextual menu next to it, it will show you a list of all your runtime folders connected to your Library.  For you, just a few. For me, the hundred or so that I have created myself.  At this point, you need to decide where you will be installing your new acquisition, either Poser 12 Content or Downloads.  Weigh this decision wisely, once you are committed and you change your mind to create external files, there is literally no turning back unless you delete everything and start from scratch.  Click Install and you're done.  At the lower right-hand corner of the Library panel, you will find this tiny icon,  , which is the refresh button.  Press it to refresh the window and you should see your new content in the folder you selected to install.