Are you getting the same old error message even though your iptables settings for OpenVZ are correct?
iptables v1.3.5: can't initialize iptables table `nat': Table does not exist (do you need to insmod?)
Perhaps iptables or your kernel needs to be upgraded.
The reason is because in newer vzctl the old way of setting IPTABLES="" in vz.conf is completely deprecated (I spent some time fiddling wondering why my settings were correct but no iptables were working).
Here is what you need to do on newer vzctl versions (there is no enabling specific or separate modules like previously):
vzctl set $vps --netfilter full --save
openvz, vzctl, update, iptables, initialize, nat, insmod, kernel, upgraded, updated, solutionare, settings, newer, quot, vz, conf, deprecated, fiddling, versions, enabling, modules, previously, vps, netfilter,