Just bought that mouse too. Took some fiddling with settings but got zoom/pan and custom buttons to work on Leopard and XP side of my Mac. Read the MX install instructions thoroughly regarding zoom. I went to the Logitech website and got info as well as updating the driver.
In OSX, I had to enable access for assistive devices in the universal access dialog of system preferences.
For PC, I don't know what I did. I set wheel button to middle button for pan to work. I messed around with settings and Zoom works now but I can't replicate what I did.
Great mouse now that it works. (I do wish it was bluetooth though)
Pete Read
ArchiCAD 12; Artlantis Studio 2
MacBook Pro 2.4 Core2Duo, 2GB, OSX(10.5) and XPpro(SP3)