Hi my ISP only provides one of those all-in-one modem/router/firewall
things for my ADSL connection, I can connect to the Internet fine by just
plugging this directly into my switch, but I want to take advantage of a
m0n0wall firewall.

I thought it would simply be a matter of plugging the all-in-one into the
m0n0wall WAN port and then the m0n0wall LAN into my switch but this
doesn't seem to work.  I looked in the WAN Interface in the web GUI but I
can't see an option for pre-configured internet.  How do I achieve this?

Any advice you can give me would be much appreciated,

Edward