If you flashed the full version of the os, you'll get a GUI when you boot the pi. Follow the guide for that:
Press <control> to get a drop-down menu. Use arrows to get to Accessories, then to Terminal.
At the prompt ($):
$ sudo raspi-config
You should already have password and network set up.
Option 5: enable both SSH (P2) and I2C (P5)
Option 7: expand card (A1)
<finish> to exit config.
Now update everything from Terminal:
$ sudo apt-get update
$ sudo apt-get dist-upgrade
$ sudo apt autoremove
If you don't have a GUI (ie, the Lite version), use the Terminal app in the mac Utilities folder and follow the rest of the directions, just include the options for changing the password and accessing the network. The default login is pi, pw is raspberry.
More info from link in Matt's guide here:
Hang in there and keep going!
[If it's any consolation, I reformatted and reinstalled various versions of the os half a dozen times. I was hoping the Guide would minimize the number of times others would have to do the same.]