From the Model Mayhem FAQ:

How do I send my Proactive Alert messages?
  1. Login and go to My Stuff
  2. Click on the Proactive Alert link in the left side rail.
  3. Where the page says "Select which Casting Call you are using" choose your casting call.
  4. Begin selecting models to send your alert to.
  5. Click "Create Alert" to begin typing your message.
  6. Send your Proactive Alerts!