OK that's probably not the right question since there must be a bunch 
of DSL'ers who are doing something similar.  I'm trying to use NAT on 
an Mac 7500 with a PCI D-Link Ehternet card as well as the built-in. 
Modifying one of the example files I now have:

macbsd# ipnat -l
List of active MAP/Redirect filters:
map rtk0 192.168.5.0/24  -> 137.78.218.53/32  portmap tcp/udp 40000:60000
map rtk0 192.168.5.0/24  -> 137.78.218.53/32
map rtk0 192.168.5.0/24  -> 137.78.218.53/32  proxy port ftp ftp/tcp

List of active sessions:
macbsd# uname -a
NetBSD macbsd 1.5_ALPHA NetBSD 1.5_ALPHA (GENERIC) #8: Thu Jun 22 
10:44:11 PDT 2000     matt@yosemite.local:/other/kobj/macppc/GENERIC 
macppc
macbsd# ifconfig -a
rtk0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
         address: 00:50:ba:43:d2:77
         media: Ethernet autoselect (100baseTX full-duplex)
         status: active
         inet 137.78.218.53 netmask 0xffffff00 broadcast 137.78.218.255
         inet6 fe80::250:baff:fe43:d277%rtk0 prefixlen 64 scopeid 0x1
mc0: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> mtu 1500
         address: 00:a0:40:20:f6:61
         media: Ethernet manual
         inet 192.168.5.1 netmask 0xffffff00 broadcast 192.168.5.255
         inet6 fe80::2a0:40ff:fe20:f661%mc0 prefixlen 64 scopeid 0x2
lo0: flags=8009<UP,LOOPBACK,MULTICAST> mtu 32972
         inet 127.0.0.1 netmask 0xff000000
         inet6 fe80::1%lo0 prefixlen 64 scopeid 0x3
         inet6 ::1 prefixlen 128
....

I can ping this box from either interface, but I can't ping through 
it from the 192.168.. subnet on mc0.  I'm fairly sure that I tried 
pinging the 137.78.218.53 address from there as well and it worked. 
(Will double check.)

Either I'm doing something obvious and stupid or else I just need to 
upgrade to 1.5.  I wanted to get a similar box working on the subnet 
first, but I can't do the install there the way I intended without 
getting NAT going first.

Signature held pending an ISO 9000 compliant
signature design and approval process.
h.b.hotz@jpl.nasa.gov, or hbhotz@oxy.edu