The driver for the USB Atmel chipset provided with these boards is not suitable for Mac environments
I hope this can save some time to other developers (at least until another version of this driver is published!).
avrdude: stk500_recv(): programmer is not responding
If you have a Mac computer and you are planning to work with Arduino Uno (or with the new Mega), my advice is to get another computer with either Linux or Windows, or you will need strong drugs...
I have just recently started working with the Arduino Uno myself on Windows 7 / 64 bit and at first I had trouble getting everything to take off and fly too. I went to the Arduino Home page and downloaded the 0022 version for Windows 7 and even after that there where hang ups.... I have to tell you that it states in the manual in the trouble shooting section that you may have problems using Arduino IDE, particularly on Windows!