I have the exact same problem as OP and own a Steam installation.
I tried the regedit trick and my keys were already correct. I downloaded the warband and napoleonic wars standalone installations from TaleWorlds and the game was running but the install for Zulu v4 would still tell me the DLC is not installed.
I'm desperate to play this mod but nothing seems to work. I'm quite certain the solution for people with this problem would be for someone to be kind enough to make a .rar of the Anglo-Zulu module and upload it to mega. That way we could get past the installer and just manual drop the files into the modules folder.
Please help! if someone could do that it would be greatly appreciated !
-------EDIT-------
I think I just found the solution! This might be a guaranteed fix for everyone.
Since I knew I just needed to get the files from the exe, I checked google for a program that could extract files from an .exe without loading it. I found this program called Universal Extractor here :
http://legroom.net/software/uniextract Once installed I dragged the zulu installer under the "Archive/Installer to extract:" path and set the destination directory to the folder I made on my desktop. It ran quickly and now I have the files I would have gotten if the installer worked for me.
Note: It's important that once you finished extracting the files out of the exe, you open the resulting folder and find the mod files in a folder called : {app}. Take the files in there and create a new folder and title it after the mod, place the files inside and then move it to your warband directory in the modules folder. It should work now, before I realized this it wouldn't appear in the modules list.
Game works now
---2nd Edit---
Game definitely works as I can make custom battles and create a character but I couldn't join the only server last night (It was the AUS server) because it said I didn't have the module installed. So now that is the problem instead of the installer not working.....