I've known this game since a month before launch, and I got to say, I love Casper. He's so bbg fr, I bought the dlcs and played them and omg, he' so so.
you have to copy/move the dlc file (it´s an rpa file) into the "game" folder inside the a date with death folder. like, when you open the game's folder, there are the "renpy", "lib" and "game" folders. if you move the dlc file there, it should work and the next time you play you'll see a "thank you for buying...." screen