To position the display window in the horizontal center and bottom of the frame, the easiest way is to position a dummy active object at the desired location in the frame, and then center the display at 0,0 from that active object.
To center an object horizontally in the display window, you need to know the position in the frame of the left and right edges of the display window, and do some basic maths.
Formula to get the horizontal center of the display window in the frame:
Formula to get the vertical center of the display window in the frame:
X Left Frame + ( X Right Frame - X Left Frame ) / 2
X Left Frame, X Right Frame, Y Top Frame and Y Bottom Frame are expressions of the Storyboard Controls object (a system object). They return respectively the horizontal position in the frame of the left and right edges of the display window, and the vertical position in the frame of the top and bottom edges of the display window.
Y Top Frame + ( Y Bottom Frame - Y Top Frame ) / 2