Lego has released Mindstorms Robotics Invention System (RIS) as a Windows-only product, but fortunately Macintosh users are still able to use the Mindstorms sets if they are willing to take a few extra steps.
RIS sets include a special adapter (the IR tower) that allows a computer to communicate with the RCX using InfraRed light. These towers have been designed for Windows computers, so the first problem in using Mindstorms with a Macintosh is to get the tower connected and working.
The second problem is that the RCX Code software (which is used to construct programs for the RCX) only runs under Windows. There are three basic options:
1) Run the RCX Code software under Virtual PC (or some other Windows emulation software)
2) Purchase the Robolab software, which is a separate programming environment developed by Lego for the educational market.
3) Use an unofficial programming environment such as NQC.