You will need to download the DLC file manually (i.e not through the Itch app), then follow the installation guide shown on this Itch page. Keep in mind you'll also likely need to locate the "14 Days With You" app, find where it's installed (likely by right-clicking), then locate its "game" folder. Since I do not use the Itch app, I can't provide much help or guarantee that this method will work for you.