Are these runtimes which already exist on your machine?  In that case it's not too hard.

Double-click on the upper folder of your librabies until you reach the top.  Should have two folders showing, Poser 7 and Downloads.  Most people don't use Downloads at all.  Click on the plus (+) sign at the bottom.

Browse to the version you want to add, and open the Runtimes folder.  Click on "Open" in the dialog, and you should now have a third folder, named Poser 4, 5 or 6.  Repeat for others you want to add.  Does that help?

John