You need to know what your LAN subnet is for this.
The default FreeNAS IP is 192.168.1.250
Most home routers have a default IP address of 192.168.0.1 or 192.168.1.1, although others are seen.
If you have the former, you need to change the FreeNAS IP to 192.168.0.250; if the latter, the default will work fine (i.e. no need to edit anything here).
If you need to change the IP:
From the initial Console Setup menu, press 2 to enter the IP setup page.
Choose not to use DHCP (i.e. press n).
Enter the IP you want: e.g. 192.168.0.250.
Enter the LAN subnet bit count: 24
Enter the default gateway: your router IP e.g. 192.168.0.1
Time for a quick reboot now.
NB: For the sake of simplicity, I will illustrate future examples and settings changes with the default FreeNAS IP of 192.168.1.250 (just substitute your own).
Ref: Novell Developer