I mentioned the MIT courses here - viewtopic.php?f=25&t=3639
- the Spring 2011 course recommends any Python 2.5.x - 2.6.x, just not Python 3.0. I think you could get by with 2.7.5.
QA - I would not attempt to downgrade Python, it's integrated into your OS, you'll likely be reinstalling your OS should you screw up.
Q2 - I would use Python 2.7.5 installed on your OS, it will work fine for most of the basics of this course. If you run into a snag, post the code that won't run using 2.7.5, I'm sure someone will help you adjust the code to make it work and I think you will learn a bit along the way.
Q3 - I have no idea about Macs but (personally) I would either find a better IDE for Mac and/or use the Mac equivalent of the Windows command prompt (if it exists) and learn to run your python scripts. I've never like IDLE on Windows, maybe it's better on the Mac, don't know.
Enjoy your class, post any questions you have I'm sure someone will be able to direct you.