Simple answer: you can either double-lick the new-home.zip file, or right-click the file and select "open."
More in-depth answer:
Okay, so first, make sure you've downloaded the DLC after purchasing it (if you haven't downloaded the DLC yet, you should be able to download it in the same manner you downloaded the game originally).
Then, you need to find the DLC file on your computer.
- If you downloaded it through a web-browser (like Google Chrome, or Safari), it'll probably be in your "Downloads" folder (open up "Finder" application, and the "Downloads" folder should be listed on the left side). The zipped filename should be "new-home.zip". Double click that file, and it'll create a folder named "post-game" in the same folder as the zip file.
- If you downloaded it through the itch desktop application, then it was unzipped automatically when you downloaded it. To find it, you should right-click the DLC in your itch library, and then click "Show in Finder". That'll bring you to a folder named "crosscode-a-new-home". Open that folder, and you should see a folder named "post-game".
The "post-game" folder is the unzipped DLC folder I mentioned in my previous post, follow the instructions I listed there, and you should be good.