1) Not missing anything. It shows up as broken because it thinks it needs a DW Insert server behavior applied for it to work. It should probably still work on the page as long as the form fulfills the IF statement trigger (may need to add a properly named hidden form element).
So the short story is you can get it to work, but it will have the red exclamation and you can't edit it.
Another option is to remove it and use validation toolkit for the validation of the unique record instead. It allows a lot more flexability and is a good exercise to learn, but is kind of complex after just crossing one hurdle not sure if you are up for that challenge just yet but you can read about it on the forums where I have walked people through it.
2) In your database set that field as type timestamp and give it a value of CURRENT_TIMESTAMP and don't set a value for the field in the insert server behavior. mySQL will automatically insert the current date and time.
Another option is to set the value in the server behavior to:
<?php echo(date("Y-m-d h:i:s")); ?>
But I prefer the database default technique.
