Thanks for posting back with the pages, this helps a bunch. When I view your registration page I can see that you are setting the id for the master table in the MRT to this value:
$WA_masterKeyValue = "".$_SESSION['profileid'] ."";
I can see the insert server behavior that you have on the page but I do not see anywhere that this session variable is being set. You will need to have this session variable set in order to make use of it in the MRT. The best way to do this would be to replace the insert server behavior that you have on the page with the DataAssist Insert server behavior. When you add this in you would specify what columns get what values from the form and also you will be able to choose the id column and have it stored in a session variable. So long as you call the session variable profileid it should work with the MRT on the page.