To add a new device to your account, log in on a new device with your ClearVPN account. In case you haven't signed up yet, you can do it in the app on the device you've used to purchase your subscription.
You can view all your connected devices on Your account page and in the apps.
In case you're wondering, as of right now, ClearVPN is available on iOS, macOS, Android, and Windows, and you can connect up to 6 devices per account.