In the library use the small arrow to "add a new runtime", then browse to the runtime and add it to the library.  If you are using an external hard drive for your runtime make sure you turn it on before you open poser otherwise your runtime won't show and you will have to add them again.

"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