Changing registration point of an mc

if you already have a simbol.. double clik on it (so you enter it for editing), then choose: Window/Design Panels/align .. click on the "to stage" icon and then select the simbol elemnts and align them as you want..

or highlight the symbol and click F8 (you will end up with a new symbol)

Click symbol on stage and press Ctrl+i (Info pane) which also displays the 9 point registration grid.

http://www.shinstudio.com/blog/2004/01/transformation_position.php