Download (save) the zipped item to your desktop(or the folder you stash them in pending installation). 
Then right click on it. One of the choices that appears is EXTRACT FILES. Click on that and just follow directions. 
You will just need to point it to either Poser 6 or the name of the external runtime you are using.   The path should read something like this; 
C:Program Files e frontierPoser 7
The file just drops into place.

The problem with the drag n drop method is that  the new file will overwrite the old one if you aren't careful., extracting just puts the new stuff in place without doing that. And if it does it a duplicate item it gives you a warning.