Here is a short guide, put together with help of some but mainly myself, on how to be able to play Co-Op with your friends over Hamachi.
You do not need any dirty settings, you do not need to disable your firewall, you do not even need to be able to ping your friends over the Hamachi network! Here’s how:
Download an older version of Hamachi, preferably version 220.127.116.11. It can be found at FileHippo. Install it, and do not bother with the license notifications and all that. Make sure you have the new Hamachi executable in your firewall exception list (if you’ve had it there before, it should still be there)!
Connect to your regular network and meet up with your friends.
Go to GameCopyWorld (GCW), select a mirror, search for world at war and find it, and in the list of patches and fixes, download the LAN fix #1. Use the downloaded patch to patch your current .exe file, and make sure it creates a backup of it (the option is a checkbox).
Open up the newly patched .exe (not the […]mp.exe file), and host a co-operative game. Please, make sure PunkBuster is turned OFF, both in your Options > Game Options and your Game Setup (inside the Lobby).
Tell your friends to search for the server and make sure they have Source set to LAN. Connect, play, and have fun!
If you can not get this working, try fiddling with your Firewall settings (maybe even try disabling it) and try playing with the in-game menu. Also the Hamachi settings might be of interest.
If nothing works, try letting the hoster host without the patched .exe and all the others connect with the patched .exe.
Also, according to a commenter, Ratza, it is important that the hoster runs his game as an Administrator by right-clicking on the shortcut and pressing Run as administrator!