From Knowledge Base Article #6703:
Note that the Instant Web Publishing (IWP) feature does not work in FileMaker Pro 8.5v2 or FileMaker Pro 8.5v2 Advanced running on Mac OS X Leopard. The IWP feature in FileMaker Pro 8.5 was designed and implemented long before Leopard, and it is not feasible to update it for Leopard.
The above should also apply to Snow Leopard and FileMaker pro 8.0. You may need to either upgrade to the latest version of FileMaker Pro or install on an older 10.4 Mac.