I bought Hoi3 boxed in a store and Dies Irae: Götterdämmerung via Games Gate.
Here is how it works on my Win7:
1. Install the game in the directory c:/Program Files(x86)/Paradox Interactive/Hearts of Iron III Collection
2. update it to version 3.05 - if its not already
3. press the win-key and "r" and type regedit or go to the registry in any way you know
4. go to HKEY_LOCAL_MACHINE/SOFTWARE/Paradox Interactive/Hearts of Iron III - may be it's located in HKEY_LOCAL_MACHINE/SOFTWARE/wow6232NODE/Paradox Interactive/Hearts of Iron III depending on Windows version
5. right click and create a new path (white paper with a red "a" on it), then double klick on it to set the value to: c:/Program Files(x86)/Paradox Interactive/Hearts of Iron III Collection
6. create another path with HeartsofIron - add value 3.05
7. create another path with SemperFi - add value 3.05
8. create another path with FortheMotherland - add value 3.05
9. If you don't follow the steps 3 to 8, the installer of Dies Irae can't find the game and says: "File exception: discfile.cpp, line: 433. Description: Could not open file".
10. NOW you can install Dies Irae: Götterdämmerung - it is VERY IMPORTANT that you unzip the data in the MOD-FOLDER!!!!!!!!! Otherwise the game will crash while you try to start it.
11. download and use the latest Dies Irae: Götterdammerung Patch and install it.
12. start For the Motherland from the Hearts of Iron III folder
13. choose Dies Irae mod - start - enjoy!
hope this will help all of you,
Best regards