From the site's FAQ:
"6. There are no participating theaters listed for me to choose from when I try to activate my code.
Try expanding your search to include all theaters within 30 miles of your zip code. Doing this may show some additional theaters that are nearby. If your certificate is for a specific movie, then the site may only show theaters that are listed as showing that movie. If there are no theaters listed, then the movie may not be showing in your area. Unfortunately, we have no control over what theaters will be showing the movie.
If you still have trouble selecting a movie theater, please contact Customer Service for further assistance. Our Customer Service hours are 8 am - 5 pm PST, Monday through Friday (excluding holidays). All emails will be responded to within 2 business days, in the order they were received."