First of all, don't install the new (Nov. 19 2005) Palm Desktop 4.2.1 for Mac if you are going to sync to a Treo 600. Bad things will happen until Palm releases a new version; read all the info on the download page and make sure the Treo 600 has been added to the list of supported devices.
For the time being, you will want to install Palm Desktop 4.1 for Mac.
If you are upgrading from a previous version of the Palm Desktop, be sure to make backup copies of up your User file (in Documents>Palm>Users). Things are going to get hairy, and losing your User file could be the element that shoves you right over the edge.
Start by opening the Palm Desktop 4.1 installer for Mac that came with your Treo 600 or with the software MacDesktop_41_T600.sit you downloaded from the PalmOne site.
Using Mac OS X, 10.3.x? Alert! Alert! Alert! The installation will choke and you'll get a cryptic system error message from the installer (not from your Mac system). Again and again and again. Don't change those settings! The problem is at Palm's end.
You could go to their Treo help pages, which will give you a number for TMobile, which will helpfully connect you to their tech support which will have no idea why you are getting the error message but will connect you by some back channel to palmOne's tech support. That's assuming you call during business hours--TMobile is available round the clock, but palmOne isn't.
You could, of course, search for several hours on discussion boards for a solution to the installer problem. Too busy? OK, here's a hint: You need to download a little file hidden on the palmOne site. It's called the palmOne Permissions Fixer.
Downloading and installing it takes but a few seconds. However, you won't be able to launch the palmOne Permissions Fixer unless you have expanded its .SIT file using the latest state-of-the-art version of StuffIt Expander. Got it? OK.
Once the Permissions Fixer is launched and runs, the Palm Desktop installer will, indeed, work.
If you are going to sync to the Treo using Apple's iSync, make sure you have the latest version of the Apple iSync Conduit (v1.2). Download it, but don't install that conduit until after you have the Palm Desktop 4.1 installed.
OK, now read the iSync instructions. The worst is over. Tell the cat it's safe to come out from under the couch.