A Biscotti ID is actually an old term used on devices that still have pre MyBiscotti Cloud Services supported software.
If your device happens to be on this software version when going through setup then it will not accept your Biscotti Account credentials. You will need to create a new Biscotti ID which must be between 3-32 characters and will not accept any special characters other than a period.
Shortly after completing setup you will see a prompt letting you know new software has downloaded and it will allow you to install this update. Once the device restarts you will then be asked to put in your Biscotti Account information (tying this device to your MyBiscotti login page) and the "Biscotti ID" will become the Device Address.