Hi,
I'm a longtime Mac user, since the early 90's. I've also been using Bryce & Poser since versions 4 (currently on Poser 7 and Bryce 6). Congratulations: once you get to grips with it, you'll have a (mostly) trouble-free life. No more security worries, and simplicity itself to run:)

I just got a new Mac Pro a month ago. It too came with Snow Leopard. But I heard about some issues, particularly concerning the creation of PDF's from Adobe software. As I use this function a LOT in my work, I decided not to risk it, and did a complete system reinstall back to 10.5.7 (which is what I was using before) before transferring my stuff across. 

However, I'm not aware of any issues with Poser or Bryce, and suspect that the reason might be as suggested above: that you're trying to install the files with the wrong expander.

Try the free 'stuffit expander' or 'MacZip': they work fine for me. I sometimes have issues with .rar files that are created on a PC: they give me the scrolling window with lots of code instead of what I expect to see. I usually avoid downloading .rar files.

good luck