Alright. I’ll go step by step on how to get the game and DLC running and see if that changes anything before thinking of other possibilities.
1. Download the most recently updated version of the base game (v5.5) and then unzip the file.
2. Download the DLC file and then unzip it in order to get the folder you will need for later.
3. Open the base game’s folder and inside you will come across a folder labeled “game”.
4. Inside this “game” folder, insert the folder you got from step #2.
5. Exit and open the game like normal. If everything went correctly, you should see a message at the top, right corner of the main menu that shows that the DLC was installed.
Note: If you have downloaded both DLCs, only install the paid version.