From a Windows box, open up a command prompt and run the following command:
ipconfig /all
This will tell you what details your windows box uses. Make a note of the following:
IPV4 Address
Subnet Mask
Default Gateway
Back on the NAS server, select option 1 again, and when you get to the following, enter the following:
IPv4 Address: Enter an IP address based on the first three octets of your Windows IPV4 address, but change the last octet to something like .100 (Eg, if your IPV4 address on Windows is 192.168.1.10, change it on FreeNAS to 192.168.1.100)
IP v4 Net mask: Enter the Subnet Mask noted from your Windows box.
The only entry you would need to enter your router IP for is if it asks you to enter the Default Router/ Gateway.
Alternatively, select yes when it asks you to configure it for DHCP, and that will tell it to get an IP from your DHCP server (most likely your router if you have very little networking knowledge). I wouldn't recommend this for a NAS, but it will work.