First, install PuTTY for Mac using
brew install putty
Then, use the following command to convert the .ppk format private key to a standard PEM format private key:
puttygen privatekey.ppk -O private-openssh -o privatekey.pem
Make sure permissions on the private key file are set properly. It should only be readable by the user that owns it.
chmod go-rw privatekey.pem
You can now use the key for logins from scripts and command line with:
ssh -i privatekey.pem user@hostname