Server IP should be the IP of the computer that's running the server software, it's so that the router can know which computer to forward to.
To check your local IP, open up a command prompt (Run -> 'cmd'), type 'ipconfig' in the prompt, and it should list addresses for your networks. If there's several and you don't know which one it is, try the ones starting with 192.168.*, if those don't work, try the others.
i use a router and modem. i have to forward on the modem. i use 10.0.0.1 to connect to router and 192.168.1.1 to connect to the modem. i have to use modem to portforward or the ports dont work. maybe that helped?