From the Model Mayhem FAQ:

Why does my Sponsored Casting Call keep telling me to select a city?
If you are having issues getting your Sponsored Casting Call to activate, and the system is telling you to select a city, be sure you are clicking the “Add” button under the city section. Without this, your city will not be added to your casting call.

We ask that you add a city like this because you can enter multiple cities into your casting call.