After a bumpy start it appears that XAMPP .04 and Zend Studio 4.02 WILL work together on OSX!
Still have not managed to hit a breakpoint and single step thru site code but I think it will happen. The Zend Studio docs are pretty skimpy.
But I HAVE got XAMPP to access the MySQL database, and my website (a dupe of one just about live access records on the db.
I still don't fully understand all about using Zend Studio and setting breakpoints (I've used many debuggers - just not this one).