Dacta, LEGO's educational branch, provides a software package called Robolab for use with the RCX. Robolab runs on Mac's and PC's, and is in many ways superior to the software included with the Mindstorms set itself. It features a graphical programming environment that is very easy intuitive for non-programmers to use. This software is distributed by Pitsco.
I strongly suggest that you consider this as your first choice if Mindstorms is your first experience in writing programs.
Robolab includes support for both the original serial IR tower and the newer USB IR tower. If you are using the serial tower you will still need some way to connect it to your Mac.
In regards to using Robolab and OS X, Deanna Rankin reports:
Robolab does not yet run native under OSX. Robolab is based off National Instruments LabView software. National Instruments says they should have their native OSX version of LabView available near the end of 2002. So I would assume Robolab will be upgraded soon after.
As for Robolab running under Classic mode it works just fine. I am using the new USB IR tower as I have a G4 without any serial ports. There were no problems setting it up to run, you just have to make sure that the USB lego tower extension is active.