You have to open both UDP and TCP protocols. If what you told me earlier is exact, the 6348 should forward to the lan IP of your mac, while 6346 would redirect to the lan IP of your PC. The best thing to do first is to set static lan IPs to your computers.
http://www.portforward.com/networkin...ic-xp-dhcp.htm
For mac os x, you should use sys prefs, network, TCP/IP -> configure IPv4 via DHCP and manual address, this is the easiest way to do it.
Bonne chance