Adding SSH private key gives error that 0644 permissions are too open
The error message is displayed because the file permission are set such that it is readable by other users apart from the logged-in user. To overcome the error message, you will need to change the file permissions for the private key such that it is readable only by you.

To do that, run the following command in Terminal:

cd ~/.ssh ; chmod 400 myKey.ppk

This will allow only your user to read (and not write and execute) the private key file and prevent everyone else from reading, writing and executing the file.

This will take care of the error message shown to you and you should be able to add the private key file all right.