You need to do following:
1. Download PuTTYGen with Pageant.
2. Press "load" button and select your ".pem" file.
3. Press "save private key" button and save your ".ppk" file.
4. Open Pageant and press "add key" button. Just all. Keep running Pageant in background.
5. Now login through SSH or SFTP without selecting password field.