For users with macOS Sierra (and I think newer):
First, you need to have an RNDIS/Ethernet Gadget interface in the Mac’s Network Preferences.
This is available as standard on Sierra. However, it might not appear automatically in the Network Preferences, and you may have to add it, using the + icon. The list of available interfaces to add will not include RNDIS/Ethernet Gadget unless the Pi Zero is actually attached.
Once added, you should see that it’s Connected, and that
RNDIS/Ethernet Gadget has a self-assigned IP address and will not be able to connect to the Internet.
You should now be able to reach it at raspberry.local, e.g. with ssh firstname.lastname@example.org.
The next step is to give the Pi Internet access; you can do this in Sharing Preferences, and share the Internet connection to the newly-established interface.
Default user: pi
Default pass: raspberry
Default name: raspberrypi