After my first attempt, I can say that getting started with Arduino is almost as easy as the directions! I have an Arduino Nano v3.0 with ENC28J60 ethernet shield and HTU21D temperature and humidity sensor module. But for reference, I did need to download drivers at Step 3, just not the FTDI driver mentioned. I needed to install a PL2303 driver. There are many other PL2303 driver choices, too: from Prolific itself, on a plugable list, and from mac-usb-serial.
My first swing at it didn't work (no blinking light, but instead an upload error). That problem turned out to be that I had plugged everything into my oldest (and smallest) breadboard. The blink program successfully uploaded and worked with just Nano, and with Nano+Ethernet, so I only had to remove that breadboard to get it working. The first troubleshooting tip I tried was "Try uploading with nothing connected to the board (apart from the USB cable, of course)." Yay, that was it!
I consider my toes dipped in the water!