virtualbox.org

I have my guest installed and running FreeNAS and I want to log in and finish doing the setup using Webmin.
I have it using NAT for now but what IP address do I use to long into the guest
Should I use Host Only mode?
If so what IP address do I use to log onto my server?

Yes, you can use the Host Only network.

Vrtual machines are just like real ones. To check the network config of a vm, use the standard tools from inside the guest OS (ipconfig for Windows, ifconfig for Linux etc).