But in the readme it said the IP of the computer i was playing at i.e. 192.168.x.x
That is the IP address range you forward the ports to, assuming your LAN config is static and the DMZ is not enabled. (and that your LAN uses that private IP range) And don't actually use 192.168.x.x but rather your private IP address as reported by an "ipconfig /all" at the command prompt (assuming you're running WinXP or NT... on Win9x use Start...Run..."winipcfg")
The IP address to enter in the sfc.ini is the router's public IP address, NOT your PC's private LAN IP address. Your private LAN IP address can only be seen from the LAN it cannot be seen from the internet so there is no sense in having SFC3 report your private IP address as the one for other players to connect to, as that would be impossible.
Have you read up on portforwarding and LAN configurations at
http://www.portforward.com yet?
Have you verified with another SFC3 player or activision that the access servers are indeed up?
Is your router connected to your PC by USB or is it a real router?