How control the textbox in as3 class

Hi there,
am having dynamic text box named username_txt in stage, i have to display the username dynamically there using as3 class. but if i use usename_txt inside the class means it throws error…please help me
this is my class

in main.fla i placed username_txt in first frame,

in first frame i created object for the class like var first:First = new First();

this is First.as :

package
{
public class First
{
public function First(){
username_txt.text=‘karin’;
}
}
}