How to setup tagged VLANs on Mac OS X

From the Apple menu (upper left corner of your desktop) choose System Preferences…

Open Network
In the toolbar below all of your network interfaces, click the gear wheel to access a dropdown menu
From that dropdown menu, select Manage Virtual Interfaces…
Click on + to access another dropdown menu
From that dropdown menu, select New VLAN…
Enter an appropriate name for your new VLAN, e.g. DMZ
Enter the desired tag, e.g. 23
Choose the correct physical interface, normally Ethernet
Click Create
Click Done
Configure your new VLAN interface as you would configure a physical interface, e.g. assign it an IP address, network mask, etc.
Click Apply
Close the network preferences window.