Brief setup instructions
Install the easytether.dmg on your Mac, make sure Settings -> Applications -> Development -> USB debugging and EasyTether -> USB are enabled (on the smartphone), attach the smartphone in Charge only or Disk drive mode (if there is a choice),
open System Preferences -> Network, click Apply for EasyTether network adapter (OS X 10.6, 10.7, 10.8) or "Ethernet adaptor (en2)" (OS X 10.4/10.5). On OS X 10.4 it might be necessary to reboot the computer.
This is the first time setup only. Next time just plug the smartphone to your computer with the options mentioned above enabled, it will establish the network connect automatically.
Download easytether.dmg for Mac OS X 10.8/10.7/10.6/10.5/10.4
Also, make sure to disable or uninstall any other third-party tethering software for Android on your Mac.