Movieclips acting as buttons questions

Question

I have 4 mc's that are acting as btns inside an mc on the main timeline
code is below
newolivefour_mc.buttonMode=true
newolivefour_mc.addEventListener(MouseEvent.ROLL_OVER,buttonOverfour);
newolivefour_mc.addEventListener(MouseEvent.ROLL_OUT,buttonOutfour);
function buttonOverfour(e:MouseEvent):void
    e.currentTarget.gotoAndPlay("over")
function buttonOutfour(e:MouseEvent):void
    e.currentTarget.gotoAndPlay("out")
there are 3 other mc/btns and all come up at different frames in  mainbranch_mc (mainbranch_mc is on the main timeline)
first question, if I add the above codes to each mc at the beginning or end of mainbranch_mc I get a null error, so the only way I do not is if I add actions to the frames the buttons start in. (is this correct?)
second question, since btn mode is true I do not need to change the mc properties from mc to button correct?
last question, since newolivefour_mc and the 3 other btns (which I did not list are all inside the branch_mc timeline) how and where do I code so the btns when released to getURL's
any help??
RD

Answer 1

1.  you can't apply code to an object that doesn't exist when the code executes.
2.  never change an object from one type to another in the properties panel.  i have no idea why that option exists.
3:
newolivefour_mc.addEventListener(MouseEvent.CLICK,buttonClickfour);
function  buttonClickfour(e:MouseEvent):void
   navigateToURL(new URLRequest("http://www.adobe.com"));
or even better, so you only need one click function for all your buttons:
newolivefour_mc.addEventListener(MouseEvent.CLICK,buttonClick);
newolivefour_mc.urlS="http://www.adobe.com"
function  buttonClick(e:MouseEvent):void
    navigateToURL(new URLRequest(e.currentTarget.urlS));