Hi Bill,
Simply re-run the installer and choose "add/remove scenery." You will need to verify your product with either a product code or DVD 1. Proceed through the screens until you come to the scenery map screen and choose the tiles for the US. Wait for the scenery to install and then go fly.
If the installer shows those tiles are already installed, it could have been that you were in demo mode. Just restart X-Plane and enter your product code when prompted or DVD 1.