Regardless of whether your platform is a MAC or PC, on the upper-right-hand corner of your Library panel there are these icons: .  Those will link/unlink the external runtime folders on your system.  In this case, the Poser 10 runtime folder would become the external runtime folder to Poser 12.  Otherwise, as you have discovered, all your old scenes are searching for files that are not connected to the Poser 12 Library.

And as DCArt posted, overwriting a new runtime with an old one is asking for disaster.