Double click on the big folder at the top of the library until the Poser 5 folder has a red dot on it.

Then click  the small Right facing arrow > 
A drop down window should appear
Click  "Add Runtime"
Browse to the location of the runtime you want to add.
If you have more than one runtime you want to add, then do the "Add Runtime" again for each runtime you want to add.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi