problem on iptables

Hi,
In our current kernel, iptables doesn't work due to the
error "can't
initialize iptables table `filter'".
I suspect that the netfilter is not turned on in the
kernel.
We download the v2.6.20.11 sourcecode and try to do a
menuconfig but
the networking packet filtering framework does not provide
any option
for us to select to turn on netfilter. Can anyone let me
know what patch files
I need to add to allow me to choose this netfilter feature?

thanks
peng