The easiest option is to use the Free Enthought Python Distribution (EPDFree). Enthought sponsors much of the development for NumPy and SciPy. EPDFree is a bundled binary distribution of Python including a set of useful packages.
It is also possible to install the necessary packages using Macports. This is one way to get a 64-bit Python installation on a Mac, which isn’t available with EPDFree.
If you’re not sure which installation method to use it’s simplest to install EPDFree.