Update your main game to the latest version (25.3.0e).
You will find an import button in the option of the game.
Select the location of the DLC RPA file (downloads probably) and wait until the game restarts by itself.
There's a black screen during restart, I'll change that when I find some time.
You can check the wiki page ( https://wiki.andrealphusgames.com/wiki/How_to_add_DLC#Ingame_import_button_(since_25.3.0e): )
