How did you originally install X-Plane? If you purchased a digital download version, you would have received a confirmation email with your product key (a 24 digit code). You can enter that number when X-Plane asks for the code number. If you purchased the DVD set, please leave the disc in the drive when using X-Plane and the message should stop appearing.
If you not yet purchased X-Plane on DVD or via digital download (or through Steam), you are using the demo and have restricted access to the simulator until you purchase a copy. You can use the demo indefinitely, but have limited scenery and flight time.