Go to this page and download the Mac version of Enthought Python Distribution (EPD):
Install EPD by double-clicking the .dmg file to mount the image.
Then double-click the .mpkg file to run the installer. Use all default settings during installation.
Spyder
Spyder lets you edit and run Python scripts.
Go to this page to download the Mac version of Spyder:
Download one of the files marked OSX based on on whether you have Lion or Snow Leopard installed.
Install Spyder the same way you install any other Mac OS X application.