In the settings for the Virtual Machine you can choose "NAT", "Bridged", "Host Only" (perhaps more) for the NIC. To access it as a server choose "Bridged".
When you first start up FreeNas the console will show a menu, one of the options is network configuration. Either pick a free fixed address on your network or let it acquire a DHCP address. You should be able to see what address has been assigned. (The installation documentation explains the IIRC.) You should then be able to access the GUI by typing this IP address into a web browser.