Glad to hear that you now have a terminal on your monitor, ready to login to. Because you are using a Development image, the username you need to input is ‘root’, and there is no password set for that user account. Obviously, this is not a secure system, but it is helpful for development and testing things quickly. The image that you flashed the first time is known as a “Production” image, and in that case, the terminal is disabled for security purposes (as you found out).
In order to reach a system via SSH, you need to know the IP Address of the balena device, and, an SSH client application (or via the command line). There is some quality documentation here that explains the process depending upon your operating system: https://www.howtogeek.com/311287/how-to-connect-to-an-ssh-server-from-windows-macos-or-linux/
Hope that helps!