The only way that I could reproduce the "isn't a valid Poser file" issue was on my Mac BEFORE applying SR4 for OSX (Mac) available from e-frontier.com. 

After applying the patch, it seemed to work just fine.  However, I also re-downloaded and installed the Kyoko Hair from Koz's website at the same time.  Again, it has always worked on my Windows machine.

So, I woud have to concur with Acadia's posting, unless I missed a step or two. I hope that you get it sorted out!  Koz has some of the best hair available!