AS 2 / attachMovie with Tweener

programming flash
1

I want to fade in a mc with Tweener. The attachMovie works fine, but tweener does not. Any ideas? Am I totally lost here?

my_btn.onRelease = function (){
    empty_container.attachMovie("myfadein_mc", "myfadein_mc", 1);
    Tweener.addTween(myfadein_mc,{_alpha:100, time:2, delay:0, transition:"linear"});
    }

nor does this work:

my_btn.onRelease = function (){
    var myfadein_mc:MovieClip = empty_container.attachMovie("myfadein_mc", "myfadein_mc", 1);
    Tweener.addTween(myfadein_mc,{_alpha:100, time:2, delay:0, transition:"linear"});
    }

no matter what path I give to myfadein_mc, tweener does not work.
myfadein_mc is properly lingaged.
Caurina folder etc. is in place, as other tweeners work fine.