Convert a putty .ppk key to an Amazon .pem file on Mac

$ brew install putty

$ puttygen key.ppk -O private-openssh -o key.pem

Or,

$ openssl x509 -in certificatename.cer -outform PEM -out certificatename.pem

Make PPK to Pem using Mac terminal

sudo port install putty
or

brew install putty


This will also install puttygen. To get puttygen to output a .PEM file:
puttygen privatekey.ppk -O private-openssh -o privatekey.pem

Once you have the key, open a terminal window and:

ssh -i privatekey.pem user@my.server.com

The private key must have tight security settings otherwise SSH complains. Make sure only the user can read the key.

chmod go-rw privatekey.pem