If you don’t have PuTTY installed these days, use Homebrew to do it painlessly. Don’t have Homebrew?

Open Terminal and paste:

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

Then type the command:

brew install putty

Then PuTTY should just work fine without any hassle. The command for converting a PuTTY Private Key would be:

puttygen privatekey.ppk -O private-openssh -o privatekey.pem