1046: Type was not found?

In my main document I have a movie clip that holds a button. In the constructor function of my class, I want to set the alpha of the button, but am getting some errors that I don't know what to do with. The path to the button is like this:

back_next_nav.prev_btn.alpha

My class looks something like the following:

public class PrevNextNav

{

     public var back_next_nav:MovieClip;

     public var prev_btn:Button;

     public function PrevNextNav()

     {

          back_next_nav.prev_btn.alpha = .3

     }

     ..... etc.

I get these error:

1046: Type was not found or was not a compile-time constant: MovieClip.

1046: Type was not found or was not a compile-time constant: Button.