OS X 10.6 Snow Leopard shipped with a new version of Tk 8.5 that is mostly unusable with IDLE and other Tkinter applications.  If you are using the Apple-supplied system Python on OS X, you should install a new version of Python that allows use of a different version of Tk.  If you are using a python.org installer, install a third-party version of Tk or the latest installers which include their own built-in copies of Tk.  See http://www.python.org/download/mac/tcltk/ for detailed information.  If this does not solve your problem, please reopen this issue and supply more information.