Poser 4 was the last version of Poser that I can recall ran on my Classic Mac OS. That was still using RSRs. I think I finally used Soft Rabbit's Mac Converter to convert them all to PNGs. Come to think of it, I did have some library items that didn't show or open until I did convert them to PNGs. And yes,tthat was Poser 6 that I had to convert them for.

Try converting the RSR to a PNG to see if it shows up in your library and you can launch it.

BTW, if you're going backward and want to use the poses in P5, you may have to convert them BACK to RSR.