After downloading the file, you need to install this campaign in your Hearts of Iron directory.
(usually C:\Paradox Entertainmnent\Hearts of Iron\ )
This MOD will modify this standard game files: almost all the files of the AI, the "revolt.txt", the "province.csv", "models.csv", the leaders files of Australia, England and USSR and the "RandomLeaders.csv" file.
Because of this, when the mod is installing it will ask you if he can overwrite those files, select "YES TO ALL".
IMPORTANT: When you finish installing it, you need to do a little change to the "text.csv" file. Search for "U01", you will find something like this: "U01; ;..." or "U01;user defined; ;..." , you need to insert just after the first semi-colon "Italian Socialist Republic" , so this will be the result: "U01;Italian Socialist Republic;...".
This must be done by you, because this file is modified by every distributor (Friendware, Koch, etc...) so the files varies from version to version.
Not modifying this, will mean that, instead of seeing in the map "Italian Socialist Republic", you will see "user defined", nothing more, so it is not critical, but it is necesary.