Networking Problem!

[Got.soul]

Hey, so heres the deal. In our block of flats we have a built in network, which carries the internet- each computer therefore has its own IP address which allows us to do file sharing over the network. So on my computer i can normally access my flatmates files without a problem. But then he got another computer.... so what we did was install a d-link wireless router onto his port in his room. This allows him- when using mac address cloning (as thats how the ISP monitors whos allowed to use the network) to connect both his mac mini (via etherent) and his laptop (via wifi) to the network. Predictably however this means that we can no longer carry on with file sharing... sort of. The thing is that from behind the router he can type in my IP address and with no problem get to my files, but i cant do the same to him- i cant figure out what ip to connect to! Im confused in general- did something like this before setting up a webserver, but that was on a belkin router. Any help would be greatly appreciated! Kyle