I solved this already, I\'m just posting it here because I couldn\'t figure it out with google, or by reading the docs. I only found it by accident.
To add a breakpo
Just adding to this answer (would've commented but for the rules that say I don't have enough reputation to do so): On the Mac you must control-click. The normal "right-click" at least for the trackpad does not work to bring up the contextual menu with the option to set a breakpoint.
Completing the answer supplied by the OP: after setting the breakpoint - you must turn on IDLE's debug mode (using debug --> debugger). When you run the program, press "Go" in the debug window that opens up and IDLE will stop at the breakpoint.
There is an open Python issue about the lack of documentation for IDLE breakpoints. Contributions welcome!
Update: The on-line documentation and the IDLE help files (for 2.7.4, 3.2.4, and 3.3.0) have been updated, with Nick's help.
You can set breakpoint before it is run.