Convert Putty PPK's to OpenSSH Key Pairs

$ brew install putty

Place the two keys in the .ssh folder in the user directory.

Now convert the PPK keys to SSH keypairs
$ cd ~/.ssh

Private key generation:
$ puttygen id_rsa.ppk -O private-openssh -o id_rsa

Public key generation:
$ puttygen id_rsa.ppk -O public-openssh -o id_rsa.pub

Set permissions for the keys:
$ chmod 600 ~/.ssh/id_dsa
$ chmod 666 ~/.ssh/id_dsa.pub

Connecting using new keys:
$ ssh -i ~/.ssh/id_rsa username@servername

Connecting to remote server using port forwarding:
$ ssh -i ~/.ssh/id_rsa -L 9001:127.0.0.1:3306 username@serverName