From the Model Mayhem FAQ:

How do I add Proactive Alerts?
If you are just posting your Sponsored Casting Call, you just check the box next to Proactive Alerts.

To add Proactive Alerts to an existing casting call:
  1. Edit your casting call
  2. Choose the Proactive Alert option
  3. Save the changes
  4. You will be taken to a review page, and then an invoice page.

You will need to pay the Proactive Alert fee in order to activate the feature.