If you are using Ubuntu part of the problem for me was that Ubuntu does not have a super user account that we can normally access all we can do is 'sudo'.
To overcome this become super user and then change the su password with the following
then this line works on Ubuntu Karmic
Code:
sudo iptables -t nat -A OUTPUT -p tcp --dport 1935 -m owner \! --uid-owner rtmp -j REDIRECT; su -c "/usr/bin/rtmpsuck -z " rtmp
You can leave out the -z if you do not want debug output.