The installation guide is shown on this Itch page (above the comment section), and on a separate page once you've downloaded the DLC. If you've purchased the DLC, you would have also received similar instructions via email.
From the looks of your screenshot, you'll likely need to manually download the DLC (i.e., not through the Itch app), unzip it, then place the "14 Nights With You" folder inside the base game's "game" folder.
Since I don't use the Itch app, I can't really offer much help outside of this.