First, associate the private key (<keyname>.PEM) with the instance to which you want to connect using PuTTYgen. Click on the Load button and browse to the location of your private key (you will probably have to change the file type to All Files (*.*)). If all goes well you will see the message "Successfully imported foreign key. . ."
Click OK, then click Save Private Key.
Click Yes when PuTTYgen prompts you about saving the key without a passphrase.
Save the key as <keyname>.ppk.
Next launch Putty to open an SSH session and tell Putty to use that PPK file -- NOT the PEM file! Expand connection, SSH, and select Auth. Click the browse button next to the Private key file for authentication: field, and select the .PPK file you just created with Puttygen.
Under category on the left go back to Session and Save the session. Then click Open to connect.
Possible error messages with incorrect credentials.
* Putty failed: "Disconnected: No supported authentication methods available"
* Server refused our key