The basic forumula for keeping something centered is:

thing._x = (Stage.width - thing._width)/2;

or, if you've precomputed the center coordinate, you can use:

centerCoordinate = Stage.width/2;
thing._x = centerCoordinate - thing._width/2;

If the text box is contained inside a movieclip, you can use the movieclip width. Otherwise, you can set the textfield to autosize and use the textWidth property instead of thing._width.

The basic forumula for keeping something bottom-aligned is:

thing._y = bottomCoordinate - thing._height;

Again, you can either contain the textfield in a movieclip, and use the movieclip height, or you can set the textfield to autosize and use the textHeight property.