Newbie-set alpha on movie

I have a box (type movieclip) on the stage, its named box_mc.
It starts on the left and moves to the right via a timeline tween.
I want the box_mc to have an alpha of 0 at the start and an alpha of 100 at the end. The alpha of 100 will (ultimately) be assigned via LoadVar.
So, basically I need to set the alpha variable of the stage item before the movie starts playing via Loadvars.
Would someone show me the code for this please.

Also, if I am moving a mc across the stage using onClipEvent (enter Frame), the mc keeps moving forever. How to I tell it to stop at a certain position, say, half way across the stage.