ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

If the screen prompts you to enter a password, please enter your Mac’s user password to continue. When you type the password, it won’t be displayed on screen, but the system would accept it. So just type your password and press ENTER/RETURN key. Then wait for the command to finish.

brew install s3cmd

Done! You can now use s3cmd.