I'm on a Mac and I have a "No server running" error. What do I do?

Launch the Houdini Terminal either through Spotlight or through Applications>houdini>current>Utilities.
Type the following commands, pressing Enter after each:

hserver -S ""
sudo launchctl load -w /Library/LaunchDaemons/com.sidefx.sesinetd.plist

You will be prompted for a password. This is your password for the machine.
Note that you won't receive any feedback about your password until you hit the Enter key.