Making duplicated movieclips into buttons

How can I get my movieclip which has been duplicated to act as a button? Neither of these seem to work:

"thumbContainer"+nextPicture.onMouseDown = function(){
currentButton = "thumbContainer"+nextPicture
currentButton.onMouseDown = function(){

It doesn’t work with onRollOver either. Is the first way an ok way to reference it? As in, will having the instancename made up of two parts make a difference? Also, I don’t know if any of the following make a difference, I don’t know:
[]It’s got a jpg loaded into it already
]It’s part of a for loop that duplicates a load of them, they all need the same code.
[*]I forgot the other thing I thought of.
[/list] [email protected] sure this is something really stupid, but I can’t figure it out right now.