From the Model Mayhem FAQ:

How do I get emailed when someone posts a casting call or notice that meets my search criteria?
Emailed or private message alerts for notices and casting calls are only available to VIP subscribers.

Alerts will send a notification with links to casting calls or notices that meet your specific search criteria.

To create an alert, simply search for what you want to find, and then click “Add to Casting Call Alert” on the results page. Give the alert a name and click submit. You’ll then get an email, a private message, or both when someone posts what you’re looking for.