For some reason, the page for this product is not up-to-date. The current version is version 4.1.1, there never was a version 1.0: Xpress Pro was first released as a version 4.0. 4.1.1 brings Panther compatibility, but only for Dual 2Ghz G5, don't ask me why single chip machines are not supported.
It's not a bad software, it's just a bit of a dinosaur (the interface) but it does its job - frequent crashes though.
To dl the latest version, you'll have to go to http://www.avid.com/products/xpresspro/index.asp and to look for the download link at the bottom of the page.