The DLC should be a folder, not a single file.
Could you please delete the "14 Nights With You" file, re-download the DLC, unzip it to your desktop, locate the "14 Nights With You" folder, then place it inside 14 Days With You's "game" folder?
Please keep in mind:
- The DLC folder must be called "14 Nights With You". If it's not, please locate the folder instead of renaming anything or using the .rpya file.
- Do not use the "14nightswithyou-free" or "14nightswithyou-paid" folder either. Instead, check inside those folders in case they weren't unzipped properly.