<<< I have just connected a Belkin G+MIMO modem router via ethernet cable to my desktop. Following the instructions I have changed my IP address and set up some basic security. >>>
The router's IP address will be 192.168.2.1 and it will allocate addresses to computers connected to it in the same range (192.168.2.x), but each computer must have a unique IP address. This is done by DHCP (Dynamic Host Configuration Protocol) and therefore, you shouldn't have to manually enter any IP addresses.
<<< I tried changing the Mac IP to match the IP address I had used on my main PC but it would not accept this. >>>
No, let the Mac get its address automatically, whether by cable or wirelessly. I don't know the specifics of connecting wirelessly with the Mac because, although I have a MacMini, I use a Belkin USB Wireless Adapter with it and some software I managed to find on the internet, but this isn't the Mac's own wireless networking. In general though, you should be able to scan for networks, find your router's SSID and connect to it. I'd disable WEP until you get the network connecting.