The method by which files are sent in Xchat, and on IRC in general, is via Direct Client to Client (DCC). A direct connection is established between the two relevant machines and the file is sent. While IRC protocol allows any non-privileged ports to be used, Xchat uses ports 4990 to 5000 for DCC.
If you were sending files to machines on the Internet, since you have network address translation, you would need ports 4990 to 5000 forwarded to the machines doing the DCCs. But since you are transferring files on your LAN, it is not necessary in this case. If all internal routing on your LAN is open (which odds are it is, although I don't know), then the files should transfer without a problem, and without need for port forwarding or anything.
To send a file, I simply right-click a user name within Xchat and send the file. To receive a file, I see in my main console Xchat window (usually) something like "Dennis has offered motd (122 bytes)". I type "/dcc get Dennis motd" and the file is pulled.
The DCC file transfer in Xchat is a little buggy, if your Xchat window crashes during a file send/receive attempt, file a bug at Launchpad. If that happens, your ~/.xsession-errors file would have be dated at or after the crash, and a line near the bottom would mention an xchat failure.