Download this TGZ archive. It should
end up in your Downloads folder. Note the file extension; it may
be .tar rather than .tgz.
If the extension is .tgz, open the Terminal utility and
enter this command:
sudo tar -x -C /usr/local -f ~/Downloads/nand2tetris.tgz
Otherwise, open the Terminal utility and enter this command:
sudo tar -x -C /usr/local -f ~/Downloads/nand2tetris.tar
The use of the sudo command on the two lines above will require
you to enter your password. Please note that you won't see any keystroke
echoes (no asterisks) appear when you type your password.