I'm not familiar with the installer used for the New York City payware you are trying to install. However, that message sounds similar to one you may see if you use another of our programs--World Editor. It appears the NYC installer needs you to show it where your X-Plane folder is installed. It should open a Windows dialog box where you can navigate through folders, drives, etc until you find and select your X-Plane install. (We recommend installing X-Plane on the desktop because it makes it easier to find in situations like this.)
If that does not work, please try contacting the designer.