Start up your virtual machine, then enter ifconfig (in the guest) and note the IP address assigned to your main network adapter. In my case this was 192.168.178.66.
Note: it is also possible to stick with the original NAT interface and SSH into the guest using port forwarding. You can read more about that here. You can find information on all of the VBox network settings in this comprehensive guide.
Starting and Stopping VirtualBox in Headless Mode
You might have noticed, working with the VirtualBox Manager and the guest OS is a bit of a pain. If you’re going to continue doing this, you should at least install the guest additions, as well as enable clipboard support.