I also got this message at first, but after some searching, i found a solution.
Put your Ipod/Iphone into DMZ mode on your router. Once i did this, i could play on multiplayer fine, but beware, when in DMZ mode, the iDevice can be a pain to connect, e.g. connecting to the internet after being in standby may not work, so i only turn dmz on for my iPod touch when i want to play NAZI ZOMBIES!!! FTW!!!
hope this helps dude