First, you will need a public key. Click "Advanced Features" in the blue box to establish your public key at the bottom of your Capsules page.
You will be prompted for a key. If you do not yet have a public key set up, then entering one will establish your key. If you already have a key, resubmitting a response in this box with change your key.
You'll find the command to SSH into your capsule in the blue "Advanced Features" box on each capsule's status page.