There is an Add Runrmrime icon in the top right of the Library palette when you open it. Click on that ( it looks like a bunch of books with a plus sign). Kt will open a directory window, browse to the folder in your library in which your previous runtime resides ( probably Public Docunents... Poser 11... ) and click that to open.

I am not at my home computer atm, but this is described in the Poser Manual as well if you look through the table of contents, there should be a How to Add Runtime chapter.