There are installation instructions in the 1.04 thread, which is the latest update. You didn't see it because its not stickied for whatever reason and is halfway down the page so I'll get a mod to sticky it. Anyways here's the instructions.
1. Ensure you have WinZip, WinRar, or some other program to open .rar files.
2.
Download version 1.04 of the mod from ModDB, open it using WinZip or WinRar, and move "Seven_Kingdoms.pack" contained in the file to C:\Program Files (x86)\Steam\steamapps\common\Total War Attila\data.
3. If Seven Kingdoms is already installed, overwrite the old Seven_Kingdoms.pack with the new one.
4. Launch Attila
5. When the game launcher opens, activate "Seven_Kingdoms" using the "Mod Manager" button (simply click the check box next to the mod). Make sure "enable out-of-date mods" is checked.
6. That's it!
If you want to download the
Battles of the North submod, you can either
download it from Google Drive (in which case the instructions are pretty much the same as above), or
download it from Steam.
I've also got
a Steam collection of all the other submods, they're not all compatible with each other though.
Also here's the troubleshooting guide if you have any problems.
The mod will
NOT work on illegal or cracked versions of Attila.
If unit names and descriptions aren't displayed, change your game language to English.
If the mod crashes at the custom battle screen:
- Make sure your game is set to the Grand Campaign, not Age of Charlemagne or the Last Roman. To ensure this, launch Attila without the mod, go to Custom Battles,
select Grand Campaign from the dropdown menu and launch a battle. After this, launch Attila with the mod enabled and it should work.
- Another method is to go to your AppData folder (try searching for %appdata% with Windows). Mine is located at C:\Users\BlickMang\AppData\Roaming\The Creative Assembly\Attila Once you're here, delete the folder "battle_preferences". This folder stores Attila's custom battle settings, and resetting it should allow it to work.
Edit: Oh yea and
here's the FAQ.