Where do you configure the tcp ip stack on mac os x?

The Mac OS X TCP/IP stack is derived from the FreeBSD stack but configuring TCP/IP stacks is more of a Windows thing than a Mac OS X thing. Depending on just what you want to achieve/what the problem is... trashing the SystemConfiguration's preferences.plist (but keeping a copy for reference purposes is probably a good idea ;-), or simply switching from a wireless connection to an Ethernet connection (or vice versa), and restarting will rebuild the settings.