Error with sample PF code

Hi,

I found some sample code in the man pf page (just scoll down to the end 
of the page, you will see it).

After compiled it and give it a shoot, it returned error:

# pfctl -sn
nat on tun0 inet from 192.168.9.0/24 to any -> (tun0) round-robin
nat on tun0 inet from 192.168.4.0/24 to any -> (tun0) round-robin
nat on tun0 inet from 172.16.0.0/24 to any -> (tun0) round-robin
rdr on tun0 inet proto tcp from any to 1.2.3.4 port = 3000 -> 
192.168.4.254 port 25

# ./a.out
./a.out <gwy addr> <gwy port> <ext addr> <ext port>

./a.out 192.168.4.254 25 1.2.3.4 3000
a.out: DIOCNATLOOK: No such file or directory

I may be have overlooked something.

Your suggestion is highly appreciated.

Thanks
Sam.