Regardless of Mac or Win, this should be same.

Open your Poser 12.

Make sure you open your Library by clicking on the Library icon (stacked books looking icon) in upper right of screen.

Find the Add Library icon that looks like what hborre posted above. Click it.

Navigate to the folder in the menu that opens that houses your Piser 10 Library (depending where you installed those these can be in your Documents under Poser 10 or elsewhere... you may need to do a search for folders called 'runtime' to locate and you must choose the folder that is a level above the Runtime folder).

Depending on how you installed content,  you may gave multiples, and in order to use older work files you created in Poser10, you need to find where those are. The work files only contain references to where the actual content is located.

I hope that makes sense a bit.