Hey, I've a simillar problem.
Yesterday I've installed m0n0wall on my old PC.
Now my network looks like this:
Linksys Router(192.168.22.1) -> PC-1(192.168.22.113) and also too Debian(M0n0wall(10.18.0.1) on Virtualbox) -> PC-2 (my PC-10.18.0.15)
My Debian has two interfaces... one connected with my linksys - 192.168.22.100
Now to my problem:
Everything works fine, but I'm unable to forward or "open" ports for my PC.
I need to open 28960 UDP for a game (Call of Duty) but after I've put in:
Firewall > NAT > Inbound > add rule >
Interface: Wan
External address: Interface address
Protocol: TCP/UDP
External port range: from 28960
NAT IP: 10.18.0.15
Local port: 28960
and saved the settings, its still blocked.
Without M0n0wall (just port forwarding on linksys) it works great...
What can be wrong?
Thanks

Pictures needed?