The link says it does not run under Leopard. That could be why the downloads don't work. From the site.
LEOPARD (MAC OS X 10.5 USERS): Thank you for your support and interest in OSXplanet. I understand that many of you have upgraded to the new operating system, Leopard and that OSXplanet does not run under it. I am working on a new version of OSXplanet but as I am a freshmen at a new university, I have decided for the first semester to focus on my studies a little more. Rest assured that I will have a new version out before the end of the year and I will redouble my efforts to focus on OSXplanet in the months following the release. However, for now please have patience and I will release a new version as soon as possible. Know that OSXplanet is not dead!!