If you look at /etc/hostconfig, there is a line for IP forwarding. Try setting it to -YES- and reboot. Note, I haven't tried this myself...
After the reboot, run (in a terminal):
sysctl net.inet.ip.forwarding
and see if it is set to 1. If not, the hostconfig setting may be ignored, and you'd have to run
sysctl -w net.inet.ip.forwarding=1
to set it.