A little more information would be useful. Are these computers on the same local network (ie. connected via the same wireless router) or are they trying to connect via the internet?
If the former you shouldn't need to adjust any router settings; just make sure both firewalls aren't blocking the application.
If however you are doing it across the internet the host will have to forward the relevant ports (there are many threads on this issue on these boards), and the client will have to connect to the host's internet IP, which they (the host) can obtain from a site such as
whatsmyip.net.