How do I centre the stage with create JS extension in Flash CS6?

Wrap your canvas in a div with the same height and width as the canvas, a margin of auto and a top, left, bottom, right of 0.

Example:

<div style=" width: 756px; height: 425px; margin: auto; position: absolute; top: 0; left: 0; right: 0; bottom: 0;">




<canvas id="canvas" width="756" height="425" style="background-color:#003366"></canvas>



</div>