I just tried unpacking the archive using Stuffit Expoander, and I get a badly-formed archive like some here have discovered. On OS X 10.3, the archive is unpacked using a utility called BOMArchiveHelper. This component seems to be a standard part of 10.3, and may not exist on 10.2 - there, Stuffit may be used by default and so it unpacks incorrectly. I no longer have a 10.2 install around to check this out on, but by manually dragging the archive onto SE, I found this problem occurred.

I've uploaded a stuffit version here:
http://www.homepage.mac.com/graham.cox/pyro/pyro1.1.sit

I'll submit an update to version tracker so this becomes the official download link