The Photosynth Web Site uses Microsoft Silverlight for its viewer and editor. Silverlight works across operating systems and across browsers, but the latest version of Silverlight (SL5) does NOT work properly in Chrome on the Macintosh. Here's the support matrix: http://www.microsoft.com/getsilverlight/get-started/install/default.aspx (see System Reqs).
The main problem that Photosynth users will see when using Chrome on Mac OS X is that entering text in the Photosynth editor does not work. That's a serious problem.
The short term workaround is to use Safari or Firefox rather than Chrome on the Mac. Silverlight does support these browsers on the Mac.
The longer term fix is for us to move more of our funtionality to HTML5, which is a project we are working on. We're starting with the panorama viewer, and will eventually get to the editor. Until then, I'm sorry to say that your browser choice on the Mac will be reduced.