Not sure why this worked in FB6 (we did change scripting engines between FB6 and FB7), but the RecipientList.Add method actually requires three arguments: Name, email address and a boolean which specifies whether this particular email address is enabled or not.
This is a change in behavior between the FB6 scripting engine and the FB7. I just checked in a fix for this which will allow the method to be called with 2 or 3 parameters (the enabled parameter defaults to true when not provided).