Find the runtime you want to use. Open the zip file so you see Documentation and Runtime.
Select and drag them to your runtime folder.
you will get a message about files in the runtime overwriting but you can ignore that. the contents of the runtime folders will install to where they need to go.

The some of the ZLegacy content is VERY old, dating back to Poser 3 or so. I'd suggest just installing the new items and playing with them before you go to the tedia of installing the legacy items you may never use.

;>

Andy!