First you need to generate new SSH key pair which you will use to connect to all Kubernetes instances
In the main menu go to "Services" -- "EC2". You will see the following screen:
You can automatically generate a new key pair, in this case AWS saves your private SSH key in a downloads folder. In this case you need to copy it to your .ssh folder and change permissions to 600. Or if you want to user your own ssh key, you can upload it with an "Import Key Pair" button.