Most installations will go straight into the correct folders under Poser 6Runtime, if you unzip to the Poser 6  folder, and they will then be immediately accessible..

BUT... 

You may want to consider setting up separate runtimes at this point, e.g. for different characters.   There are various threads in the forum about that and I am certainly not the best person to offer advice on that subject.   While my separate runtimes work OK,  I do have to go through a rigmarole similar to the above when I install anything new.   I generally install to Poser 6 and then move the files from libraries to the corresponding locations in my separate runtime, while leaving any Geometries or textures folders in their locations as initially installed.   

The advantage of separate runtimes is that it speeds up Poser quite a bit in the creative stage (it won't affect render times).  If everything goes into the same runtime, you can end up with a Pose folder, in particular, which takes an age to load and is a nightmare to navigate.