It took me ages to get it to work.
I eventually had to manually remove all references to the previous Sony Reader software version. Just trashing the app doesn't remove all the files.
Once I did that, and then downloaded and re-installed. The app worked, though as in a previous post it seemed to take a long time before it recognised I had a reader as it kept asking me to register. (That might be a server issue though as it is the Holiday season ?).
Anyway, the Sony reader, V3.1 on Mac OSX10.6.2 is now working for me.