First, you will need a public key. Scroll down to the blue box at the bottom of your Capsule's status page.
You will be prompted for a key when you attempt to SSH into your Capsule. If you do not yet have a public key set up, then entering one will establish your key. You can do this by clicking Add or Update Public Key in the blue box. If you already have a key, resubmitting a response in the field below 'Public Key' will change your key.
You'll find the command to SSH into your Capsule in the blue box on each Capsule's status page.