if you talking about centering a display object on the center of the stage then it you can load the flvPlayback into a Sprite that has it registration point on center then centering that sprite then it's like
mySprite x = stage.stageWidth/2;
mySprite.y = stage.stageHeight/2;
The way you wrote the question it sounde like the video is full screen. If it's fullScreen wouldn't it have to be centered by definition?
almost certainly an align issue.
stage.align="TL";// so that the reg point remains in the top left after resize
myPlayer.x = ((stage.stageWidth-myPlayer.width)/2);//similar to what you wrote, but using stageWidth, this will also work when flash isn't at full screen
myPlayer.y = ((stage.stageHeight-myPlayer.height)/2);
Thank you all for taking the time to post and help out with my issues. I entered Mark's code and it worked right away on both of my screens at different sizes and resolutions.