From the Model Mayhem FAQ:

How do I add a verified credit?
  1. Go to My Stuff
  2. Select “Credits” from the left side navigation menu
  3. Click “Add Credits.”
  4. In the text box that appears, type either the member’s username or their Model Mayhem number and search
  5. Choose the correct member if more than one person has a similar name or Model Mayhem number
  6. You can also request credits based on your Credited Photos!


  7. Once you've selected the correct person, add your details and submit your request
This credit will not be published until the other user verifies the credit in their credit notifications.

As noted, you can also request credits based on your Credited Photos! Clicking “Request Verified Credits” will prompt the system to scan your portfolio for active members with credits and generate a request invitation that is ready to be submitted to the matching member(s). You can also request to modify the details and request individual credits from members you have already worked with.