Dynamic Text Problem

Hi all, I am trying to change the text of a dynamic text field when I hover over a shape so far I have the shape and a dynamic text field called myText_txt.
But can't get this to work

Actionscript Code:

on (rollOver){
    myText_txt.text ="Isolation";
}

Can someone please tell me why? the rollover is on the shape and I would eventually like to have several shapes.
Cheers