To SSH into your Amazon AWS EC2 instance with the private key that was downloaded when the instance was created you can use the PuTTY SSH client. PuTTYgen must also be downloaded to convert the private key to a format that PuTTY understands.
To convert the private key first run PuTTYgen and import the downloaded key by going to Conversations -> Import key.
Once the key has been loaded you should see a screen similar to the one below. From here you can click the Save private key to save the key to your PC in a format that PuTTY can read. You may want to include a passphrase as well to help secure the private key.
Next run PuTTY and in the left hand menu click on Connection -> SSH -> Auth. Under Authentication parameters specify the private key that you generated under Private key file for authentication:
Then in the left hand menu again click on the top most item Session. From here you can enter in the public DNS address of your Amazon AWS Instance and click the Open button.
Next you will prompted for a username which should be
ec2-user. If you
exported your private key with a passphrase you will also be prompted for this.