How to Use _parent in AS3.0

Hi Friends,
in AS2 we used to _parent and _root.

but how we can use this in AS3.0

I have a problem…

I have a button “nextBtn” and a MovieClip “animMc” on the stage.
on first frame of main stage i wrote “nextBtn.visible = false”,
when animMc play and reach to last frame, nextBtn should be visible.

(in AS2, I can use “_parent.nextBtn._visible = true”)
how can i do this in AS3 ??