Console (always works): select the VM and click Console. It opens in a separate browser window, where you can also send key combinations, take screenshots and download the log.
SSH (Linux): requires cloud-init and OpenSSH in the image. The default username depends on the OS, e.g. ubuntu for Ubuntu or centos for CentOS. Log in with the SSH key you selected when creating the VM – cloud images have no default password.
Windows: use the username set via Cloudbase-Init; without an SSH key you need a password.
📖 Full guide: Connecting to virtual machines